I'm not sure that really gives confidence hardware has really slowed down enough to invest in it for decades. Single core CPU performance has but that's not really what new things are using.
Of course, similar to a 10 year old car or appliance, you will be missing any new features or bells and whistles that have become available in the meantime.
My NAS is about 13 years old, the network switches it connects through are even older, and while 2.5GbE now exists I have no need throw out my "good enough" equipment to replace with something marginally faster or more power efficient. I don't even really need to expand the storage of that NAS anytime soon, because my music collection could never come close to filling it, my movie/TV collection isn't growing much anymore due to the shift to streaming, and the volume of other stuff that I need to back up from my other computers just isn't growing much over the years.
Like the PC in the 80s starting to eat up "get a mainframe" or "rent time on a mainframe" uses.
Of course one can always upgrade components piecewise as requirements change, but I don't see why you need to invest in a big ass server to do that. It'd be cheaper to go that route everyone has for decades at this point - upgrade with normal sized stuff as needed and not try to make it an up front multi-decade home investment out of it.
On the flip-side, if you intentionally plan to lock in the capabilities to the kinds of things one can run today and know you'll never therefore need to upgrade it then you can get whatever sized system makes sense for today's needs. You just need to be really sure you'll not be interested in "the next big thing" when it comes too.
Another data point? All the GPUs i’m looking at buying for my home llm explorations is +5 years old.
AI models are changing every other day. I have to rebuild llama.cpp from source regularly. We are no where close to a personal "AI mainframe."