> those gpu's will be obsolete in 5 years, but will the newer be enough better as to be worth replacing them is an open question
Doesn't one follow from the other? If newer GPUs aren't worth an upgrade, then surely the old ones aren't obsolete by definition.
My bellybutton fluff, uninformed opinion is that heat cycling and effective cooling are probably a much more limiting factor.
If you are running a gpu at 60C for months at a time, but never idling it (crypto use case), I would actually hazard a guess that it is better than cycling it with intermittent workloads due to thermal expansion.
That of course presupposes effective, consistent cooling.
Anecdotally, I killed two out of two that I was hobby-mining on for a couple of years. They certainly didn't sound like they would work forever.
There is the question - will they be worth the upgrade? Either because they are that much faster, or that much more energy efficient. (and also assuming you can get them, unobtainium is worth that what you have).
Also a nod to the other reply that suggests they will wear out in 5 years. I cannot comment on if that is correct but it is a valid worry.
MTBF for data center hardware is short; DCs breeze through GPUs compared to even the hardest of hardcore gamers.
And there is the whole FOMO effect to business purchases; decision makers will worry their models won't be as fast.
Obsolete doesn't mean the reductive notion you have in mind, where theoretically it can still push pixels. Physics will burn them up, and "line go up" will drive demand to replace them.