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.
To me, it’s a percentage game. OP’s computer is now 90% a different computer, your phone example is 20% a different phone, and your car is 30% a different car. The Ship of Theseus is 100% a different ship when it is fully swapped out, no matter how long it took.