If F/1.4 is supposedly "too wide" of an aperture for astro, just wait until the complainers discover "stopping down the aperture to improve image quality.." I know the fixed apertures of the 600 and 800 F/11 are confusing y'all, but come on.
Jokes aside, having a wider aperture means stopping down is even better quality. F/1.4 might be rough for astro, but I bet the F/2 would kick the butt of the 14mm f2.8 at 2.8.
I think this lens would be incredible and probably make a profit off of high customers, like the 28-70. I think the 11-24mm was great proof that there's plenty of customers that have been spending 6-12K on super telephotos that would easily spend 3-4 grand on "super" non-telephoto lenses. That mentality has gone into the 28-70, and I'm sure this lens would do the same.
I personally think this would be an incredible sports lens. I know one of the main clients of the 15-35 f2.8 is sports, and this lens would definitely be sold in droves for sports photogs who already shell out 12 grand for supertelephotos.
A side note, the new tracking AF on the R5/R6 means you could have facial tracking on a remote camera. F/1.4 could be usable even on remotes if the camera is tracking faces.