If the 9700K is in the same price bracket as the 8700K then I will probably be buying one for my next computer upgrade. Why? Well, according to the rumors, the 9700K will overclock to 5.3GHz on air while most 8700Ks overclock to around 5-5.3GHz after de-lidding with water cooling. This shows that the soldering instead of thermal paste is worth it and that hopefully the 9700K is a bit more efficient in relation to voltage requirements for overclocking. To be honest though, if they had a soldered 6 core K-variant in the 9x00 generation then I would be considering that over the 9700K if the pricing was right.
As for those who claim "a i7 2600K is good enough for modern GPUs", tests do show that "good enough" is dependent on how good your GPU is. A 7700K at 5.0GHz is actually slow enough to bottleneck a GTX 1080 ti at resolutions up to 4K depending on the game. The CPU needs to be fast enough to not bottleneck the GPU until the GPU has hit it's peak performance. With the imminent release of the RTX 20x0 series, the older i7 CPUs are just not going to cut it if the performance of the GPUs is a good 20%-30% over the 10x0 series as is claimed by Nvidia.
*edited* fixed a brainfart with the 7700K clock speeds lol