The 9600K stutters like a skateboard down a flight of stairs in a couple AAA titles at 1080P on an RTX 2080TI, i know one might say that's an unreasonable setup but when talking about longevity it's not, it's going the same way all the other i5's have, the 7700K isn't quite there yet, why? SMT.
The 9700K, a £380 CPU has two more cores, threads, 8 of them, the 8700K will outlive the 9700K as useful high end gaming CPU, hell my 3600 will outlive the 9700K.
The 8700K was and still is a very good CPU, the 9700K already isn't.
Intel, as always give you "just enough" on the day just one step below the highest end, the 8700K was the only exception to that rule.