In my eyes adding a HDD isn't wasting thoughts but something you'll do sooner or later anyway, so why not do it ASAP? It won't hurt and you won't need to waste any more thought after that.
I would just add one as soon as I need one. With a 8GB one that would be pretty immediately, but for me a 32GB it has plenty of space for the few games I play and then I don't need to take care to loose the USB or accidentally format it. I try to keep all the games that are important to me in the internal memory, to reduce the risk of loosing them and their saves when the console gets packed away for many years.
As long as the original MLC is still readable or you have good MLC + SLC backups and want to reflash SLC, too.
I am sure we will soon be able to rebuild MLCs. If a eMMC dies by writing it should be read only, so still readable anyway.
I never ever saw a GPU or CPU failing that easily and I putted them under a lot of thermal stress.
Thermal stress wont kill them immediately, but over time. Especially the thermal cycling of the different materials, breaking joints over the time. Given the wii u is after the time of the underfill problems, that plagued the xbox 360 and PS3 and the Wii U has probably good margins. And if the high side mosfet fails and sends 12V to the CPU you would also get a dead cpu.
Also heat makes the data leak out faster from the eMMC if you are concerend about that
Also pretty sure the Wii U does an emergency power off before reaching max tolerated temps (never checked this but it would be stupid to not do it).
I am not aware of any temperature sensors. I know they turn off with a red blinking led if they overheat to much, but that could just be something no longer probably working. Even if it has an emergency power off, you don't want to use it just below that probably pretty high threshold if it could be running much cooler.
In other words: I'm able to use really old, heavily used CPUs or GPUs without issues but flash storage with the same age? Locked readonly (or worse) since decades already.
Some resistor or something near the CPU might burn before the CPU gets damage...
My first SSDs are still going strong, I only had two sd cards failing. But I also have failed Graphics cards here and a dead Athlon...
//EDIT: And the Wii U is known to do MLC writes even while doing standby functions only (title update checks and so on). So while I also never looked into that I'm skeptical when someone sais there shouldn't be that much writes anyway.
But you won't prevent that with external storage. And do we really have evidence, that the wii u is writing excessively? As far as I can tell this came only up as speculation, when the first hynix started to fail after a long time not being used and with the data we have now, we are pretty sure there was just something wrong with these chips.
The console with the worst eMMC I had so far looked from the inside like never used, not a spec of dust.
If we assume the MLC can sustain 10 000 then that would amount to 320 TB total writes on 32GB console. How big is the part of the save game, that gets regular written? Maybe double that amount to account for FS overhead.
So I am really not that worried about wearing out my MLC. But on the other hand, I am not playing that much and I would have no problem replacing my MLC if it fails. So maybe take that with a grain of salt.
