> the new parts probably won't work in the old machine

Except with framework, where you can actually upgrade it piecewise. The CEO had a video showing of them doing it in like 10 minutes, part by part

so in 8 years, I'll be able to buy a new CPU and it'll work in that old laptop?

Quite possibly yes. The level of upgradeability they've given their Framework 13 line over the years has been very impressive, and you can still put the latest CPU in the original chassis if you want. https://www.youtube.com/watch?v=JSxgCEpkiKM