Depending on your performance needs I would somewhat agree with the above.

I have a D750 and Fuji XT1. Basically if you don't need high shutter speeds in low light and can do without decent AF focus tracking then mirrorless may be a weight and side saving option.

I use the D750 when I need cracking focus performance and also when low light coupled needs to be coupled to this. But if these requirements aren't there I can get just as good shots, sometimes preferred shots (mainly due to glass) on the X~T1.

While it makes sense to get another FF body to complement your lenses, you would be somewhat duplicating the D4 with the D750. Which depending on how you stress the D4 (ie fps and buffer maybe being key factory?) could the D750 replace the D4?

Personally I love my D750. It is a performance beast vs the xt1. I don't even bother trying to capture action with the Fuji anymore

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