Others have gone and said much of what I would but I will go anyway. It is a subject I have tracked and contemplated somewhat over the years. There are a variety of factors are play from what I can tell.
1) Hacks available. If a simple hack appears for the newer models then older ones have limited value, especially if the older hacks don't confer much. If newer hacks have more limited abilities or a serious list of annoyances then the total control afforded by older hacks changes things -- some might contrast the DVD vs JTAG/RGH stuff on the 360 but I reckon it would be more like if the sky3ds (an only runs 1:1 ROMs flash cart) is compared to the firmware level (homebrew, cheats, region free, hacked ROMs, possibly free...) hack appeared. As has been mentioned the pool of viable targets is quite large, and not really shrinking at anything other than the usual rates of hardware destruction (more on that later). Easier mods are nice but it is not like the 360 JTAG stuff or something where newer hardware revisions blocked it and so did updates that most people would have done (and continue to do -- compared to PS3 3.55 that any number of people could pull out of a cupboard then the JTAG stuff dropped while the 360 was still getting amazing games), and the harder mods are not that hard either -- some slightly tricky soldering but nothing a basic electronics repair setup will not be able to handle.
I don't imagine all the hacks will suddenly be abandoned either -- sometimes for lesser hacked systems you have one person doing all the work and if they drop off the map then sometimes nobody picks up their mantle. I similarly don't imagine we will see long term blocks in newer firmware updates or newer games struggling to work.
Also in this take the time to dump the NAND, keys and anything else like that which is necessary and either stick it on an SD card, old hard drive or DVD in the same box so you know you have a copy of it. Might also be worth emailing it to yourself if you can.
2) Whether it is banned and the nature of bans for the system, including whether 3rd party alternative stuff is available. I don't imagine Switch bans will do as much as 360 bans did for some people (with the DVD hacks there then following each banning wave the COD COD and more COD set saw several people try to flog their "useless" 360s for less than the price of a game when the second hand price at the time was far higher) but at the same time I reckon it would probably be more of an issue to some than it was for the 3ds or Wii. Also at the same time you will probably find a significant audience that don't care to pay Nintendo to get p2p servers on Splatoon and will happily take a hacked/hackable Switch off your hands for their single player needs.
3) The value of a system as a whole. If the Switch dies on its arse and does not get any games then it will go down accordingly. I can't predict the future here but I am not expecting sunshine and rainbows/land of milk and honey -- you might get some Nintendo efforts that people crow about for a couple of years, some half hearted ports (or ports of older games), a few indy titles of note (which will probably get immediately remade on everything) and a third party effort might defy the odds but we are not looking at another PS2 or DS here where there will be exclusives and a library that ascends into legend*. I don't imagine homebrew will add much to the Switch compared to say XMBC and emulators for the original xbox -- too many devices nowadays you can take with you or plug into your TV to do as good or better than the Switch likely ever will to make the Switch unique enough to matter here.
*that will require Sony, Microsoft, EA, Ubisoft and phones, possibly also PC, to completely lose the plot and nobody to snap out of it in time to pick it up and run with it. While Sony and MS are making some questionable moves and Nintendo are actually making some good noises I don't expect it will amount to much in the long run. Phones might also just about break out of the "apple is good stuff" rut they have been stuck in as far as games are concerned.
4) Any hardware flaws that are discovered. Included in this are whether your battery still works -- leave a lithium ion battery powered device in your cupboard for a few years and the battery will probably get into deep discharge and need replacing (maybe a still hard to do recovery will work but for the cost if you are not able to do it yourself with gear you already have a replacement will probably be easier) but that is easy enough to add something to the calendar a few times a year to charge the Switch back up. I don't know if we will see a PS3 YLOD or 360 RROD equivalent here -- it is a hot device, with known flexing issues, with lead free solder and using fine pitch BGA gear but at the same time I am not quite seeing the early victims in numbers I would expect to see if it was going to be a serious and widespread issue while the system is active (nor have we seen it in the Shield which is much the same hardware), even accounting for the "3 months on and my Switch is just a dust magnet/paper weight" thing. While increased use tends to decrease life then sitting in a box is not the same as sitting in a stasis field -- room temperature/hot attic over years is still a problem for dodgy solder joints, though you should be spared screen rot over such timeframes. For example any older (pre Jasper) 360 motherboard, even if it is prime JTAG material, will not get a lot for it as RROD is considered inevitable at this point.
This also has knock on effects for unbanning as any unbanning with official stuff will likely be the result of a working Switch giving up its key then unlike the 360 there will not be the option for someone to buy a stack of 20 RROD boards, give them enough CPR to boot up for the 30 seconds it takes to snatch the keys and get a stack of keys to use in this.
As it is not a lite Switch you should also be spared any great issues with dodgy controls compared to say bad power slider on the DS lite or dodgy shoulder buttons on anything with shoulder buttons, even more so if it is relatively new and in a box.
5) Whether a new hardware revision appears that obviates it -- while I like my original DS I am apparently a freak and most people won't consider anything other than a DS lite or newer. On the flip side people said the same about the original GBA and now we have the option to stuff a nice AGS-101 screen in there they seem somewhat desirable. The newly announced Switch lite will probably not be it but if a Switch XL or Switch+ or whatever comes out then that might change things, even a new screen might change things (if we suddenly get a really swish IPS or OLED...). I don't imagine we will be seeing Switch XL exclusives or just plain 100 times better on XL of note either but that I am prepared to be wrong on.
In any case if you were thinking of doing it for an investment that returns a lot of money then don't. Right place at right time and someone will probably drop something for it in the "*drops some notes* get it here within the hour/before the evening is out" type scenario. If you just plan to stick it on whatever the 5 years from now equivalent of ebay is or on the selling section here then there are easier ways to make £50, especially over the course of a few years -- if you want to do it in games sell it and buy some more limited run and rare games of quality and future interest and sit on those instead. There is even that company that does limited runs of otherwise download only titles. What those games might be is harder to guess -- sealed Nintendo first party is not the worst plan but those are more hold value with minor increases than increase dramatically, exceptions being for things that suddenly get popular (Fire Emblem on the gamecube was "oh wow it has that guy from Smash Brothers" and sold 50 copies to people that like strategy games, today though after it rose up and became as crazy popular as it is the prices for GC Fire Emblem are obscene). Low run RPGs of quality are not the worst bet (especially if they are the de facto best version of that game) but reissues and downloadable options here change things a bit. I would not try to guess which random collector item will be it -- low run sports games, exclusive to pack in double packs, early version uncensored game, games pulled from shelves, speedrun fodder and more all feature here in addition to the "only released in Europe a year after the thing died" type releases. That hybrid nonsense also means I am not sure what will happen for the handheld vs home console thing -- plenty of things are rare on the PS3/PS4 but fairly available on the Vita and that can change things a bit.
