12.9in is definitely not an obstacle for pro apps... (as said above, it's bigger than 13.3 16.10). And if they ever make a larger iPad, it will be at the very least 14in. No point in making it only slightly bigger...The one reason I have that Apple is working on larger screens for iPad Pro is that they can’t get some of the Pro apps to work well on 12.9/11”.
13.3” is smallest screen size of a Mac. If Apple is indeed working on migrating Mac Apps to iPadOS, the existing screen sizes are probably too small.
A 16” iPad Pro will never happen, however I can Apple going for something in between. Between 13”-14”.
Also personally I don't think they'll kill the 12.9, but rather add a new model with its own keyboard accessories... The real point is if they manage to make it light enough.... especially given that Samsung 14.6in one is going to be lighter than the M1 12.9...