I have 5600x with rx 6800, and I'm at playing 2560x1440. I get what you are saying, unless I go for one of the new gpus, changing the cpu will not bring the same percentage of improvements as seen in reviews using rtx3900ti.If you want the best on AM4 platform, yes.
Otherwise 5600x can work just fine up to rtx3080.
I dont want more than 3070 or 3070ti perf for this platform, PCIE 4.