People say 'It works, don't bother replacing it' but I don't see that in the car market at all.
I treat my car just like my PC, tablet and phone (mostly), keep using it until it breaks (or a large lump of concrete in the road breaks it for me).
But, I see people with new cars that had 'nearly new' cars all the time, and cars cost a lot more than PCs. New cars have some innovation but it isn't worth the price (to me at least). I think it is because they are mostly 'rented' and people just see the monthly cost; the lease expires and they get the next one. This may explain why companies replace PCs of course, they often lease them. But, the issue of retraining for a new OS means a lot of extra cost which may prevent changes.
The main issue for PCs is the one everyone mentions, PCs are just not required as much any more. I use (this) laptop (second-hand, c £500 but quite nice) for the usual stuff but connect it to a telly and it does a lot more (no chromecast etc. required). I use it for some of the Linux work I do too when needed (courtesy Oracle VM). Most people do not do even that.
Many people bought a tablet and immediately removed a large part of the need for a PC. Phones already were making PCs less needed.
Most people's PC still does what they need, because even iPlayer and other video streaming etc. requires almost no CPU power and little GPU power.
Few people edit their videos or need high power Photoshop type programs. Word (et al) runs easily on an ordinary machine.
Since no-one else 'sees' their fancy new PC, people will not treat it like a car (a status symbol), hence way more iPhones in evidence than makes sense from an economic point of view. This also explains why 'cool' all-in-ones and super-slim laptops sales hold up well - you can show them off in coffee shops and University cafes.
There is no chance of a new OS 'making' people upgrade. It is hard to show it off for starters.
We would need a true AI, super-clever device that needed some new hardware and OS to work; then people might upgrade. Don't hold your breath.
Meanwhile, people just buy a 'smart' speaker and get the same power, cheaply, cause 'the cloud' means someone else runs the powerful machine for you.
At least PC makers have the opportunity to shrink without too much pain, diversify, make similar but different machines (Smart Speakers!) and supply the fixed, corporate use that (I think) will continue for the forseeable.