The only reasonable APS-C advantage is the price.
If we stop right there, I agree. You also stated, "If 5D3 had 46mp sensor, then you could crop 18mp image out of 46mp and it would be just as good as your 7D image and much better when you don't need to crop it," which is tehnically correct, but practically irrelevant since the 5DIII does not have 46 MP, nor does any currently available FF dSLR. By your logic, if the Canon 5200mm lens was the size of an 400/5.6 lens and could be mounted on a Phase One IQ180, and that Phase One could shoot 10 fps at ISO 25600, I'd have the perfect birding setup. See how 'if' is pretty unhelpful, sometimes?
You can see that in my previous post I was talking about D800 vs D7000 and there are no if-s.
I switched to 7D vs 5D3+imagination, because the person I was replying to (Bob Howland) is using those two cameras. I also stated that "most of APS-C advantages are 'synthetic' and $-related". Not making 4.3μm FF sensor is a 'synthetic' 4.3μm APS-C advantage. They have the technology, but they decided not to use it, because they would make more money this way. Why? - Because most consumers prefer speed (high ISO and FPS) over resolution. 20-22mp may be enough, but they shouldn't stop there. We must wait till they figure out how to make both sides happy. Highest resolution + more powerful CPUs + pixel-binning may be the answer.