If you compare 9600K, 9700K, and 9900K all at 5GHz you will find that few games see a serious performance. In fact, almost universally you are better off spending the money on a faster GPU or tighter RAM kit.

AMD supports their sockets for 3 generations of CPU, while Intel does 2 on their consumer chips, and lately only 1 on the HEDT. But a single generation still lasts 3-5 years easily.

R5 2600 is really popular because with it can match or beat an i5-8400 for a little less money. But as you go up the stack, Intel adds clock speed while AMD mostly just adds cores. Games scale less with every additional core, while clock speed is almost perfectly linear.