For strictly gaming the 9700K is more than enough. You can get by with any competent 6 core CPU for gaming only(maybe even 4 core as my 4790k would suggest). Going higher than that means the OP has money to spend. In that case, why not future proof his PC with hardware that the next consoles will have, albeit with less cores and clock speeds? I believe when Intel really strikes back at AMD(with a 7nm or less architecture) the current Lakes will be obsolete.
Games will become much more demanding in a few years from now. Currently, games target 28nm Kabini CPUs running at 1.6-1.8 ghz. Even my 4790k can handle every game today without any problem. But what will happen in 2-3 years when PS4/Xbone games stop being produced? Games will target an 8 core 16 thread ZEN2 chip at 3.2ghz. Add in the worse optimisation for PC that requires even more hardware to run.
Bottom line is this, if the OP only cares about CURRENT 2019 gaming only he should just buy an 9700k. It is overkill already let alone a 9900k or the 3900x. If he cares about the future, not that far future but in a few years, better get the 3900x, and just upgrade to a new GPU then, rather than buy a whole new system again(he will need a new motherboard, CPU , probably RAM as well if DDR5 becomes a thing). Assuming the OP really cares about running the games on the highest possible settings that is.