People are not going to take PS5's and reuse them but if it was legal to build products around PS4's reuse you can bet your ass a sizeable amount of PS5 users would love to make use of their old hardware. It would take a while for killer apps/products to release but stifling it before there's a chance seems like a huge waste to me.
I understand your pessimism but in this case I ask you to dream a little more.
The fact that people aren’t already doing so with regular general purpose computers seems to entirely discredit this theory, no?
There's a massive second-hand market for many general purpose computers. Plenty of people, myself included, buy used hardware almost exclusively.
Beyond that, many people are creating home theater PCs, home labs, home automation systems, arcade cabinets, custom car audio head units, drink robots, and more using commodity hardware. Often times these are reused hardware, which cuts the cost on hardware by a significant margin.
Just because you don't see it happening doesn't mean it's not happening.