I have shot motorsports, news and fashion with a variety of bodies including the D3s and D750. I recently shot a race and wanted a crop-sensor body to partner with my telephoto, for just the reason you say: I found I was doing a lot of cropping with the full-frame body, and instead of longer glass wondered if the D7000 (at 6 fps) might work.
Losing those 3 fps was agony. It was the same when I shot a fashion event with a D750. Though I loved the AF in the new body, and the fabulous ISO performance, when I reviewed the photos I decided losing the fps wasn't worth it to me.
I think it's going to be up to you to decide what you want or value most based on your style. For what it's worth, I see one of the guys from Getty using the 5DIII, and that's 6 fps. His stuff goes into Vogue and WWD so he doesn't miss the fps. He also shoots with just the 100-400, working f/8 at 5000 and up. I carry a bag full of primes, and shoot f/2 or wide open, standing right next to him. My point is I think you will have to decide for yourself what works for you. And I still see pros with the D300s at the track, for example, though high ISO performance is obviously rarely an issue for that sport, and instead it's the native floor that's more an issue.
If you're finding that you're cropping a lot with the 400, just as an alternative, have you considered a teleconverter? I realize you may not want to give up a stop for night games, but it could be worth a try. If you're happy with the D3 otherwise (and I love my D3s), a TC14 might be worth a try.
I am needing more megapixels than my D3 is giving me, due to my cropping. I am shooting night football with a 400 2.8 lens.
Just looking at a chart, it appears the D750 would give me double the MP and better ISO, while only sacrificing 2.5 FPS. Am I missing something ? Or would this be a good trade up ?
Gordon