I personally can't see them dumbing down the D750 replacement that much, as Nikon still include it in their "professional" line of cameras.
The recent cameras in the DX lineup that have had questionable specs are very much enthusiast cameras - so I can see Nikons logic to an extent here, to create separation against the professional grade bodies.
I can see them killing off the D6xx line though, and having the D7xx, D8xx and D5 in the FF lineup.
A replacement is heavily rumoured for this year, but at the end of the day - if you need / want to upgrade now, the camera will still be as good as it is now, even if a new one comes along with fancier specs.
Worth bearing in mind the replacement will probably be considerably more expensive, where as D750 prices won't fall that much more, as they've already come down a fair bit since launch.