At 4K resolution you will be almost entirely GPU limited even with the 2080Ti. As such there is basically no benefit in upgrading over an i7 6700 just for gaming at that resolution. The 6700 is still quite fast for gaming even if you were running a lower resolution monitor with high refresh rate eg. 1440p 144Hz and you probably wouldn't see huge gains with a CPU upgrade in that use case either, likely not enough to really justify a new motherboard and CPU.