the D7500 and D500 make sense if they are seen separately from previous models. I doubt we are ever going to see a true D7200 replacement.
I feel we will see the same with the D750 'replacement', I can honestly see the D750 and D610 being merged into one new body (whatever they call it) which only has one card slot. Canon do 1slot/2slot with the 6D2/5D4 line up so why not Nikon? It makes sense to differentiate between entry level full frame (that's what the D750 really is), mid level full frame (d810 level) and the flag ship full frame (D5 level). 1 slot- entry level, 2 slots- mid level/flag ship makes sense to me, there has to be reasons to move people up a level and spend more. If you don't like only one slot buy the next step up with two, same goes for AF, fps, etc. Nikon have totally lost their way with numbering/model differentiation, it looks like they are starting to work it out.
How many D810 sales have been lost because of the D750? You could of course say how many have been converted to full frame? Part of me thinks the D750 like the D700 were rare beasts in that their spec was higher than they should have been considering the rest of the product line.