IMO the only reason to get a different camera is because your current gear gets in the way of doing what you want.
I shoot FF, but cropped sensors just couldn't capture the dynamic range I needed. I could exposure stack to get around that. They also aren't that great for night sky photography. Yes, I have a 'tracker' so I can go past 15 seconds without stars becoming streaks - but then my foreground is all smeared and I have to composite a sharp sky with a sharp ground.
Now I mostly don't bother with bracketing exposures, and 15 seconds is plenty for night sky with a decently fast lens. The
D750 is 'ISO-less' (meaning ISO is just a gamma knob) so I can stay at ISO 800, raise everything in post, no worries about blowing out anything bright.
I guess if I was going to switch systems I'd pony up for a Fuji MF - but my FF can do what I want so that's not going to happen.
BTW, if you like the Nikon 50mm 1.8D, you'd like the 1.4D even more. Same era, same coatings, better mechanism and IMO better bokeh.