Currently my rig is RTX 3070 i3 12100f (upgraded my first gaming PC that originally had a 2060). I can play most singleplayer games at 60 fps and less heavy games at 120-165 Hz on 1440p. I want to play games with RT but the 3070 seems to be not enough for this at 60 FPS ultra (/ optimized) settings. At least TW3 is not very enjoyable at 45 fps.

Now, before I will buy any new GPU, I might make sure to have a good enough CPU to handle the GPU. I want to stick with Intel since I already have a decent B660 mobo.

But here's what confuses me:

A) When I look up comparison videos on YT the 12600k (or 13400, both game almost equally good) makes up only a difference of ~10-15 fps to a 12100. For a price increase of more than 150€. What's wrong there? Also if the i3 manages to do 120 fps in AAA games, shouldn't it be able to manage even a 4080 or maybe even 4090 card, even more so with DLSS3 Frame generation? Why do people recommend to not do so? Most people say that i3 12100 3070 is the max combo.

B) How about RT impact on the CPU? When playing TW3 Next Gen Update, I realized my CPU bottlenecked my 3070 (25% usage) maxing out at ~45 fps. If I want to play with RT for the next few years without buying a new CPU, how good should it be in order to do so? Or is TW3 really THAT bad optimized and it is not the fault of too little power of my i3?

If the i3 might be fine for now, what should be stopping me from buying (just as an example) a 4090 now and a better CPU one day, or maybe never if it works? I want to understand the importance of the power of CPUs. Because why on earth would anyone pay 400% price increase for maybe an i7 over my 120€ CPU if it does it's job fine? I am speaking specifically of gaming at at most 165 Hz 1440p, because everything above is just not worth it for casual, passionate gaming in my opinion.

Thanks for enlightenment!