Battlefield 1 and other Frostbite games don't care about clock speeds after a certain point, PUBG is an early access game that doesn't even merit consideration.
You are assuming the kind of games that the OP might play and not looking at the overall picture. Has he said that all he plays is Arma 3? Sure then I'll happily recommend him the 8700K and tell him to overclock. No, he specifically said that he plays pretty much anything and everything - so it makes absolute sense to tell him about the average picture, which is that a non-K 8700 will not be more than 5% slower than the 5.0GHz 8700K, at 720p.
Which is basically a non-issue, no matter how you try to spin it by egging him on to the unlocked chip, when he's clearly said that he's going to game at 1440p for the next two years.
You won't convince him. You have to know that OP is still on 60Hz and he already has i7 4790K, which is decent CPU. So best CPU for OP? On 60Hz?...
IF OP decide to go with 144Hz then i5 8600K or i7 8700 may be very good decision. He will also need good GPU.
I think biggest transition would be from 60Hz to 144Hz, if he stays at 60Hz then every CPU upgrade is nonsense. With GPU upgrade and decent monitor he could simply enjoy new games even on older CPU.
