Richard,
Like you I had a D700 and all I wanted was a replecment for it. In 2013 I bit the bullet and bought a D800. While i was a bit sceptical at first the quality of the files was amazing, even though I had to tighten my shooting discipline. I kept the D700 as a backup but I hardly used it. Until i had the D800 fail (mainboard failure), send it to Nikon for repair. When the D810 came out I waited for the price drops in the UK (I was living there at the time) and swapped both D700 and D800 for a new D810. With all the small improvements I think, in hindsight, was a good decision. I really like the D810 and won't part with it. I don't mind having two cameras and the D810 file quality is amazing. Whilst I could never really trust the D800 Af system like I did the D700 AF, I can say that with the D810 I'm totally over the "D700 replacement syndrome"
The D750 seems that would be a very good companion to the D810. I wouldn't give away any meaninful iq, could use the same lenses, batteries, chargers and will have a similar AF system. Also raw files are more friendly to edit than Fuji.
It's not that I'm thinking I'm doing something wrong,my doubts come from regreting selling it as for some situations it is a good system. On the other hand the opportunity cost of keeping the Fuji is not having a second Nikon body, with all the benefits as stated above. Is more optimisation of photographic resources if you will... Even though I could go on and buy the D750/D500 without selling the Fuji, I would probably fell "guilty" for accumulating more gear than i need. To be honest I already have more gear than what I need, so doing that will make it even worse.
I don't care about mirrorless being the future, as previously stated, I don't think I will ever (ever is a long time I know
) be without a DSLR. Maybe the future of mirrorless is CaNikon
!!!
About the D750 what is your limit in terms of ISO? I've been getting good reasults up to ISO 6400 on my D810.
Thanks for your time.
Regards,
Paulo