Upgrading part wise on PC is very overstated. Yes you can buy a faster Vid Card, better ram, Faster CPU. But its all tied to what the motherboard supports. If you have an older PC, just putting in a faster vid card often doesn't improve things that much. PC gaming performance is a sum of its parts. A slow CPU or RAM is going to hold back a vid card. Putting in a fast CPU with an older vid card doesn't really get you much either. And its all limited to the Motherboard as when new Ram and CPU architectures come out, you have to change out the MB to support them. Your best performance upgrades come from upgrading the Motherboard, CPU, Ram and Vid Card all in one go. And for less than an extra $100 you can put it all in a new case with new power supply. So noticable performance increases are really just brand new PCs.
On Xbox I would assume you would just buy the new model, not parts, wouldn't make sense.
What do you mean by left behind? I build a new PC, I can still play Doom on it.
I can also buy the latest game on my older PC and turn some settings down and play it fine. This is the model MS is aiming their console plans for.