Old stuff can still be used for smaller tasks... proxies, backups, smaller and less demanding front-ends... unless the power bill starts creeping up.
Upgrades depend, but mostly blockchain stuff does not creep out over time, its by experience the other way around. Often the technology advances faster. So, upgrading is usually because you want to take over other challenges.
Like for example, me, is trying to run the FULL HIVE node from home (currently I only run a witness node which is a sort of a minimalistic version of the full node), in parallel with Hive Engine that I already run.
But this time was also because I want to move to DDR5 and NVMes are getting interesting to do some cool stuff at home. Hence the move... and because I didn't want to wait more time. Has been quite already a wait for this.
CPU's are not the main reason for upgrading, they are actually in the bottom of the reasons in the list, and often is the part that hangs quite solid for most of the time.
Crucial is usually always memory speed/bandwidth and then IOps... those are the motivators nowadays. Then for whom plays games or wants to do AI, it will be the GPUs or for AI, the specific accelerators (even if GPUs can do the job too, there is cheaper and more powerful stuff out there for AI).
Anyhow, all part of the job stuff... 😇