The end of the DS line? No. The beginning of the end for the regular style DS line, yes. But I think we regular DS users will be OK for at least one more year before the game releases really start to dry up for us.
I suspect the very first thing that will signal the end of phat/lites will be that all games will start being AT LEAST DSi-enhanced, if not DSi (and 3DS)-only in the future, and there will be very few DSphat/lite-specific games being produced anymore. What we'll be left with is gimped DSi-enhanced games to play on our phat/lites until they drop the "enhanced" part and go DSi-only. I say gimped because obviously you won't have access to those DSi features on a phat or lite, or even on a DSi with current flashcarts. I believe this moving away from DSphat/lite games to be inevitable. It's what I'd do if I were Nintendo to drastically cut down on the software piracy issue.
All I know for sure is that I'm not buying ANY of Ninty's future systems until there is some kind of hack or flashcart that supports ALL the features of the newer systems, be it DSi or 3DS or whatever. I'm not happy with all the online-centric gaming, nor do I like the idea of updating a firmware in a handheld toy (especially when it can disrupt my "free-ride"), not to mention the how I despise the overload of cutesy-crap games, RPG's and pure shovelware that constantly gets released on Nintendo systems. I'm tired of wading through 100's of worthless game releases to find ONE game that I might actually like.
As for the term "DS" going obsolete......meh, it's just a name, I don't really care. If the newer system is going to provide backwards compatibility with standard DS software, then it doesn't really matter what the new system is called, it will still have everything necessary to play those older games anyway. Once the new system comes out and becomes established, people will then treat the DS like people treat the GBA now, so the DS going bye-bye will soon be forgotten anyway.
Unless the hackers really get cracking on the DSi (and the 3DS when released), then our little free-ride is almost over. And for me, no free-ride means there will be no ride at all because I'm certainly not going to BUY a system that has no hacks or flashcarts and every game I want to play will have to actually be payed for. Screw that noise!