There's about 3 reasons I can think of when it's time to build entirely net new, that may have happened at everyone's point in PC building:
a) Your power supply went and totaled your system
This was probably a bigger issue in late 90s to early 2000s unless you were about buying power supplies that would protect your components if the PSU surged and died.
b) Your PC has completely fallen below minimal requirement specs, GPU and CPU. Usually if you've waited this long, you're going to be swapping out a new CPU which will force you to get a new mobo and you'll need a new GPU anyway, possibly better ram, I guess it depends when you got your DDR3. Then if you don't have a SSD, it might be good time to get one.
c) You are building a PC for a very specific purpose. For instance gaming PCs can be a bit Frankenstein, you're looking for the most performance for the least amount of cost. But building a PC for serving, or rendering etc, something that's has a specific function other than your day to day. You'll want to put that PC together differently, things like power requirements come to mind because your PC is on all day etc.
That being said, answer to your question is probably (b). I'm actually at (b) right now. PhenomII X2 955 (unlocked to X4) with a Geforce 660, that I didn't get enough use out of. I'm just waiting for the latest hardware specs and proper DX12 FL12_1 cards to be within that price/power range and I'm overhauling my entire system. If you get the top of the line parts however, your system lasts a hell of a lot longer with no additional upgrades required.
Luckily the consoles serve as a decent stop gap (for their price) and will for maybe a couple more years.