Here's my thinking re: HEDT - They'll still be capable in 10 years time of that I have no doubt, so if I got a 3960x setup (which I looked into) I'd still be using it in 2030 without issue, just matched to a really good GPU from the time. However, the VFM of a £1200 CPU vs, for example, the much regarded in here 3700x at just £300 can't be ignored but that really relies upon the 8C/16T 3700x (matched with a really good GPU again) still being as capable in 2030, just as my 2010 2600k with a 1070 still is in 2020 for my 1080p gaming. Who knows if 1080p gaming will even still be there in 2030, or will it have died out and 4K then be considered as "basic" as 1080p is now when 8k/16k is around (will it be, who knows?) etc.
If I was looking to change the "core" parts (CPU/MoBo/RAM) of the build every 3 or so years (which may end up being a better plan) it'd be much easier to decide. However my "keep the core the same & just update the GPU" plan, which worked well for the 2600k, is probably too dependent on the current tech having another stagnation period, where something like CPU's upto 16C/32T (either Intel or AMD) stays as the maximum core count for "desktop" setups for at least 5+ years. Plus with the PS5/XBox X I hoping that makes all game devs (inc PC) fully utilise that 8C/16T by 2024 at least, meaning that CPU core count becomes a limitation rather than the GPU being one. I mean what core count will PS6 etc be using in another 6-7 years time?
Or on the other hand, could Core count be less important and just IPC be the big development steps instead, so Desktop chips top out at that 16C/32T mentioned above but the IPC improves 15%+ every 2 years instead?
It's just so much guesswork with so much changing over the past 2 & next 2 years that my worry isn't so much that whatever I get will be bypassed in 3+ years (I know it will be so why worry) but that there's a technological "jump" in say 5+ years that'll leave 8C/16T trailing in it's wake forcing me to upgrade the "core" anyway making all this "decade planning" a little moot
I suppose this discussion is helpful to me in any case TBH.