I just replaced the last parts on my 20-year-old "PC of Theseus."
To be honest, the case is still the original one, but almost every other part has since been replaced. Now, I’ve taken it back to the shop where I bought it 20 years ago and asked them to upgrade the motherboard, CPU, and memory - the last of the original parts.
So, is it still the same computer?
I also like that I can just keep replacing parts on an existing product rather than buying an entirely new device each time. That's exceedingly rare feature these days.
I used to do a lot of building, modding, overclocking, etc. I can't tell you why, but I always associated the motherboard as "the computer." If I replace the CPU, RAM, cards, cooling, drives, case, etc it's the same computer. And if a take a mobo out and put it in another chassis, that's now "the computer" or, at that point, "the old computer."
I had one 3/4 tower case that lasted me from 486sx, all the way to Pentium 3 and I still miss it, but I wouldn't say it was the same computer. The same case sat next to Moss's desk on The IT Crowd, and I'd get a little nostalgic seeing it.
I feel the same. The motherboard determines what else you can fit in, like the chassis of a car. It determines the maximum GPU, CPU, ram, etc you can use.