2. Considering that a D810 image is 7360 pixels on the long edge and the D5 image is 5568 pixels on the long edge, if you make a 60" print from the D810, then a D5 image would yield a 45.39 inch print at the same resolution. Considering that probably less than 0.00001% of all images get printed larger than 45" inches you can draw your own conclusion as to how beneficial the additional pixels are in the grand scheme of things. Full Disclosure, I own and love the D810.
I'd consider 20-24MP good for around 20x30" or 24x36", and marginal for larger prints. I'd consider the D810/A7r as being excellent at 24x36", good at 32x48" and marginal for 40x60", although the larger format is more forgiving (e.g. where I might want 200ppi for a smaller print, a larger one may only require 150ppi, or even less at the very largest sizes). It's not a strictly linear relationship - as the print size increases, you tend to need more overall pixels, but fewer PPI. Ideally, I like to produce 40x60" prints using stitched images, even with the A7r2 (although a 70-80MP sensor would likely change this), but would still be happy to produce one from a single frame if that's all I had. I couldn't say the same about blowing up most 21MP 5D2 files to that size, particularly low-key images or ones with lots of shadow details.
You don't often need to print that big, but, with the right image, it can be the difference between, 'Here's $5k, I want that thing blown up huge to hang in my lounge' and 'Sorry, I can't print it that big and still have it look good'
Bottom line is, for 99% of applications/shots, any of the D810, D4s or D750 will be more than adequate and will give you indistinguishable final results. But, when you're comparing top-tier gear, it's all about getting that last 1%, where the different cameras will give you noticeably different final results.
A top athlete would do a lot to shave 0.1s over 100m.