Thanks to less megapixels, there is less noise per each pixel. It has been a thing for ages in any digital camera equipment but nowadays people seem not to see it because of high image noise reduction algorithms.
No disagreement with this.
Probably the last major advance for noise was BSI sensor technology, which lets the pixels be a tiny bit larger(for a given pixel density) since it cuts out the interconnects between pixels. I seem to remember there being some other advantages that reduce noise.
You'll find this beaten to death on photography forums, but one of the things that is demonstrably true across pretty much all makes is that around about 2008-2010 when 5-digit ISOs became "normal" or "expected", there was also a pretty dramatic shift in color rendition/discrimination. I think Nikon navigated it somewhat more gracefully than Kodak did, but I've done the test shots myself that show that color discrimination is worse between say a Nikon D2X and D3s. Those aren't 1:1 comparable, since the pixel density is MUCH higher on the D2X than the D3s(same resolution, different sensor size) but I find the D2x more or less intolerably noisy past 400, while the D3s is comparable up to 1600, and I still think it looks better at 12,800 than the D2X does at 1600. Probably a more even comparison is to look at the Kodak SLR/n and the D3, especially since both are similar resolution full frame CMOS sensors. I consider 400 a practical limit for the SLR/n(the older DCS 14/n...well don't go over 125 or so) where D3 looks great a LOT higher. At the same time, the SLR/n colors are basically everything Kodak use to know that made their color science the best in the business.
Somehow or another in all of this, too, Nikon managed to pull off some color magic in the D3X. The camera has its issues, and I know it was a joke to a lot of people at the time because of the price and "no one needs 24mp", but the colors really are like nothing else I've ever used(and bear in mind that I tolerate klunky Kodaks just to get the color).
Even though I have...well...way too many cameras, though, if it really got down to it I'd be content with nothing but my D5. I've had the thing 2 years now, and after initially hating the size, after making myself use it for a week or so, I've not put the thing down. It's my default camera 95% of the time, to the point that my D850 has nearly made it to Ebay a handful of times over the past year. I'm not in the market for another camera now(famous last words for anyone who knows me) but I'd be just as likely to buy another D5, or get a D6 while they're still available, than anything else. I do use 2 cameras a LOT, but depending on the situation my second camera is either a D4 or D3x, and the D850-which should fill the "second camera" role for me-is a distant 3rd on the list.
