Yes emulation needs fast single-threaded performance. It seem you checked that yourself and if 2600 is good enough, go for it.I know when it comes to emulation, I need good single thread performance and for PS2/Wii/Gamecube emulation, the 2600 seems to be fast enough to run most games at 100 % speed. I can get the Ryzen 2600 for only $160, whereas the 9600K will be $280. I also like that AM4 motherboards will support AMD processors through 2020. My thought process is that I would spend less on a CPU now and in 2 years, I would upgrade to whatever the mid-range AMD processor is then. As far as I can tell, in terms of performance per dollar, this makes the most sense for the next 5 years but I'd still like your opinions on which processor would suit me better.
What i disagree is with upgrading. Forget it. Whatever is out in 2 years time from AMD will not be significantly faster single-threaded than a 9600k and if that is the case it would be visible in price. Spending $160 know and another $160 in 2 years is still more than $280 for the 9600k and with the 9600k you get the performance now and not in 2 years. Or said otherwise upgrading the CPU is simply not worth it after such a short time. Better to buy a better one right now. If you don't trust the 2600 is good enough that you need an upgrade in 2 years, get the 9600k. You can easily save the money on the gpu. A 1070 is overkill for 1080p 60hz. Get a 1060 6gb or RX 580.