Practically this doesn't often happen, especially on work machines. All the parts are loosely linked by generation. It's not often you want to jump forward 5 years in GPU tech but not want an upgrade in every other field. Unless you are swapping parts out yearly for the absolute latest bit, there is not a whole lot of benefit over selling your existing device and getting the new one where you will see improvements across the board.
Okay, what happens if something fails/dies? At least it's replaceable by the user. It's very common to upgrade the RAM capacity rather than buying it all at once. Many CPUs have stuck around long enough to see GPUs replaced. Hard drives get swapped out for faster/more space.
I love the idea of component machines, but at this time of my life, I would be just fine to never have build another PC. With that, these systems of plug in the cables and go to work are appealing. Having said that, it still rubs me the wrong way on how non-upgradeable these things are.
It's true. But then if you are only interested in swapping RAM or GPUs, you can keep your machine for a long time.
None of that swapability is available on any of the Apple systems, so when they stagnate there's nothing that can be done.