Most stuff in a PC never goes bad or breaks. I think the most common parts are the GPU and the PSU. CPU's, memory and motherbaords tend to just work forever in my experience. But yea if you upgrade it so often or manage to keep breaking stuff and you eventually replace everything then yea it's a different PC sort of. As long as you don't replace the CPU and motherboard you don't even have to install Windows again so technically I would say you can repalce everything except the CPU and the motherboard which are the two parts you likely don't need to change. Most modern CPU's are very powerful and will last at least 10 years.

I'm still on a 5820k for example and it's still playing new demanding games with no problem. It's the GPU that I change most but I've only gone from a 680 to a 1070 in that time because one of the VRAM modules was going bad.

If you replace components as and when you need to over a long period of time is it a new PC or is it just the same one you have always used? Like if one year I change one part eventually you won't have any of the original parts but does that really change anything? You original Windows install might not be around but you can still use you're install image to a new PC. It's an interesting thought at least.