Another advantage of hobby anything is that you can just do, and reinvent whatever you want. Sure, fast CPUs/MCUs exist now and can do whatever you want. But if you feel like reinventing the wheel just for the sake of it, no one will stop you![1]

I do think some people that remember fondly the user experience of those old machines might be better served by using modern machines (like a raspberry pi or even a standard pc) in a different way instead of trying to use old hardware. That's from the good old Turing machine universality (you can simulate practically any machine you like using newer hardware, if what you're interested in is software). You can even add artificial limitations like PICO-8 or TIC-80 does.

See also uxn:

https://100r.co/site/uxn.html

and (WIP) picotron:

https://www.lexaloffle.com/picotron.php

I think there's a general concept here of making 'Operating environments' that are pleasant to work within (or have fun limitations), which I think are more practical than a dedicated Operating System optionally with a dedicated machine. Plus (unless you particularly want to!) you don't need to worry about all the complex parts of operating systems like network stacks, drivers and such.

[1] Maybe we should call that Hobby universality (or immortality?) :P If it's already been made/discovered, you can always make it again just for fun.