You should read some of the many reviews out.
Intel i7-9700K vs. 8700K
i7-9700K 9th Gen CPU Review
Intel Core i7-9700K Review
Intel Core i7 9700K processor review
You fail to take into account a few things.
- IPC gains per series
- API Cycles typically 2-3 years
What do all of those reviews have in common if gaming is your only concern?
Diminishing returns above a 6C/6T CPU
Will API's every make use of anything you can toss at them? Yes, but you are 1-2 cycles away from that so if your a gamer that only wants max settings and max FPS then regardless you are going to look at upgrading your CPU and GPU every 2-3 years.
As far as what is better between a 9700K and 8700K well I would actually say a 9600K or one of the new Ryzen chips offers a much better value for the money.