My upgrade path is pretty weird, I tend to make a grand gesture by upgrading to the best I can afford, then making GPU upgrdaes along the way to keep the system relevant for gaming. Back in late 2011/early 2012, I built a pretty solid rig:
i7 3960X @4.2Ghz
Asus Rampage IV Extreme
4x 4GB Corsair Dom Plat 2133Mhz
2x HD7970 (followed byt 2x R9 290X, followed by 2x RX VEGA64....no reduced to 1x RX VEGA64 due to lack of CF support in games)
Seasonic X-1250
This rig was my main gaming rig, but is now my 2nd rig (having 6C/12T really helped with its longevity), it's hooked up to my Samsung 49" Freesync 2 monitor and it's doing a damn nice job handling games even now.
Now, this year has been a Ryzen year, and I couldn't help but feel it was time for a new build, again, I went with what was available back in June, so I jumped right in. I was in Toronto at the time, so I had to buy parts piecemeal, but finished my build when I got back home in August, this is my main rig now:
AMD Ryzen 9 3900X
Gigabyte X570 Aorus Xtreme (AGESA 1.0.0.4)
4x 8GB DDR4 XPG D60G 3200CL16 (have a set of 4x 8GB Patriot DDR4 3733CL17 RAM incoming, my niece is gonna bring it to me when she visits me here)
Samsung 4TB 860 EVO + 2TB 860 QVO + 2x 1TB 850 EVO + WD 6TB Black HDD
Corsair HX1000 Plat
PowerColor RX VEGA64 Red Devil (Collected this RMA'ed card while I was in Toronto, still a useful card for 3440x1440 gaming, awaiting NAVI23 or Ampere)
As you can see, I'm more or less in line with the belief that getting the most cores available at the time and upgrading along the way helps ensure the longevity and usefulness of a DIY system. I'd expect my R9 3900X to last at least 3 years or more. Gonna skip AMD/Intel for the next few years while I try to eke out as much performance outta my main rig...