>I thought there was a way to mount a drive without a letter these days?
You've been able to mount drives to subdirectories of existing drives since at least Windows 7. So yes you can certainly have more than 26 drives.
It might confuse poorly-written software, though. I remember Steam won't let me install a game under C:\Stuff because it thought C: didn't have enough free space, even though C:\Stuff was a different drive with plenty of space.
I thought there was a way to mount a drive without a letter these days? But I've never run out of letters, so I can't say I paid much attention.
Windows 10 is clunky because of lack of attention to detail and a drastic reduction in testing without a reduction in scope, not so much because of backwards compatability. Windows Phone 7 lost Google because of no backwards compatability though (they had excellent CE apps, I'm told).