Today, the cost of storage is neglectable and processing time is (almost) the same.
I don't agree with that. On my 8-month old laptop, changing from one 45MP file to the next and zooming in takes 10 seconds if the power saving performance reductions are in effect (to conserve battery life). If I'm outside of access to external power, in practice I can't use the full power of the computer because it would run out of battery too quickly.
For me the limit of a good browsing experience is a 1s switch between images in 100% zoom. I understand if you spend a lot of money on a computer, you can get that, but since I have no use for the extra pixels, I'd rather not spend an excessive amount of money on a technology which loses its value so quickly (and is kind of fragile, motherboards, hard drives etc. fry relatively easily).
With 20MP and 24MP files, I don't have to wait at any point. That's to me preferable. Going through hundreds of thousands or even millions of 45MP NEF is the worst kind of nightmare to me.
500000 45MP NEFs in three copies, each copy with 2:1 redundancy takes about 195 TB of storage; that would be just too expensive for me.
On top of that, edited 16-bit TIFFs with 2 layers would increase that storage requirement even further.
You can set up your D850 to 24Mpx (RAW M).
Yes, you can, but it will be softer than the output of a real 24MP camera. I don't care for pseudo-raw raw files, it is just not a good idea.