Are you thinking about most people who use a computer or most people who use a CLI based editor that's presumably on a remote machine they just sshed into?
Hardly any of my colleagues knows what POSIX is (and surely not in any depth, even those that do), but they still SSH and use editors that include Vim all the time...
I'd say in a company of 50+ SSHing people, around 5-6 know POSIX and its history, and usually the older ones (35+).
Especially a 2019 user, 20+ years removed from the systems, decisions, and rationales, behind POSIX.
Just try to get someone (even a seasoned Linux user) to use a POSIX-only userland (as opposed to GNU), as see how fast they'll be pulling their hair out...
I would say it's not just a POSIX demand. Most unix tools are POSIX-complaint, and the user should expect just it and nothing more. Non-proper text files without LF at the end may not be processed properly, so to be assured that everything works fine that LF is mandatory.