I have been thinking over the same issue for well over a year now: what to do if (we now know: when) Canon does not bring out a capable successor for my 7D Mk II. The last 2 years, 2 people from my photo club went from the 7D and 7D Mk II to the Nikon D500. Since then I have done a lot of comparing in identical situations (shooting airplanes standing shoulder-to-shoulder) and have seen that the D500 is a lot more capable than the 7D Mk II. But ... that only shows in specific situations.
When the lighting is good and the subject's contrast is okay and the subject does not move erratically, there is only a limited difference.
But when the lighting is dull, and when the subject has not much contrast and in particular when the subject moves erratically, the AF-system of the D500 is noticeably better than that of the 7D Mk II. Just comparing the specs is nice, but you actually need to see where and when the D500's higher specs matter.
The D500's sensor is also quite a bit better than the sensor of the 7D Mk II. The DR is 1.8 stop better at low ISO. Again, in perfect lighting that does not matter. But this becomes noticeable when you need to correct for e.g. too deep shades or a bit overexposed skies in post processing. Many on this forum keep repeating that DR is of no importance for a capable photographer - implying that canon does not need to improve on that feature.
In my experience that can be true only when you can control the lighting. Usually, in action phoography that is not the case. I have seen quite a few times where my photos after post processing (for correct aircraft color and exposure) had a bleakish sky, whereas the photos taken with the D500 still showed a blue sky. And also I have had quite a few situations where after post-processing the shadow side of the plane on my photos was very dark and without much detail (otherwise noise would become apparent), whereas the photos from the D500 could be made to show some detail and better colors.
An important reason to disregard the Nikon D500 could be that is quite a bit more expensive than the 7D Mk II. And comparable lenses from Nikon also are more expensive. So if it has taken a long time to buy the lenses you need from Canon, than changing brand to Nikon may be no option.
For myself, I will wait what the "90D" will be like. I do not expect too much, because if the AF of the "90D" and in particular if the degree it can be customised would de the same as the 7D Mk II, Canon would have named it 7D Mk III ................