You will never be able to future proof computational power in the current paradigm. IMHO, you are better served by building $1k PCs three times as often, or $1.5k PCs twice as often, as a $3k PC. The diminishing returns really hit you hard at that level.
Pretty much this, in a year or two, much cheaper hardware will come out, although we are rapidly approaching the physical limit.
Depending on what the use case of this build will be, since you are still a little bit far off from getting the whole system, I’d get the AMD Ryzen 9 3800X ($449)[16c/32t, 3.9Ghz base, 4.7Ghz boost, 125W) or Ryzen 9 3850X ($499) (16c/32t, 4.3 Ghz base and 5.1Ghz boost, 135W).
I switched from Intel to AMD once I realize how cheap and effective their Ryzen series are.