You might, and I emphasize "might" find that having a 24mm to 120mm one-zoom option is actually viable--mainly due to the D750's better-performing focus system, as well as the constant f/4 aperture. The earlier 5D classic I had with the Canon 24-105 f/4 L IS USM had a very weak AF pattern (the TIGHT, 9-point diamond), and a weak AF module with f/4 glass; the D750 has gone wayyyyy above that, in terms of being color-aware, has a lot of focusing strategies, has negative 3 EV capability, and AF sensors that are sensitive even with very small-aperture lenses or lens+ TC combos...
I know you're planning on ditching the 24-120, but it's got a LOT more top end, meaning a lot fewer lens swaps.Using the 70-200 means you are absolutely forced to swap out the lens every single time you want anything wider than 70, while the 24-70 means every time you want a medium tele, you must go to the 70-200; the 24-120 bridges a huge span of useful focal lengths. And now, you will have class-leading high ISO capability, and a much stronger focusing option, even with f/4 glass. Just something to consider I think.