As for why is good low light performance important ?
If you are shooting in many different scenarios where you have no flash then low light performance is very important.
If low light performance isn't really important then many people can still use 6 years or older cameras at 100ISO and still get amazing results.
I think many would like to get the best low light performance possible from their cameras, if anyone thinks good low light performance is not important and shouldn't be at top 5 important things on their want list then good for them, to each his/her own.
I don't think anyone would argue that it's not important; simply not the most important for everyone. I know you did not mean to suggest it for everyone, rather it's possibly the most important for you in most of your shooting situations.
When I first started this photography thing, I watched lots of videos (after I purchased my 60D). I can't remember his name off hand, but the guy reminded me of Santa Claus

. Several times in his video he said it was pretty tough to buy a bad camera these days. It's what I would tell my son (often a reference or target when giving certain kinds of information or advice).
In 15 years, your D750 will still be a kickass camera; the images you make with it now won't suck because they were not made with whatever camera Nikon comes out with two years from now. How you use it will likely still be the most important thing. Somewhere around here, there is a pretty doggone good photographer still using a 5DM1.
A general question or thought - at some point will there be diminishing returns with sensor technology? In some folks minds, the gap between crop and full frame in some respects have gotten closer and closer. The decision between the two is not always clearcut until you figure out what/how you shoot - mostly.