Honestly I cannot imagine who expects the original vi and trusts vi to be there. Every Unix/Linux user I have met expects Vim and trusts Vim to be there. If there are users expecting original vi, they must be a very small minority.
A standards-conformant implementation of vi is absolutely required to be present and conformant to standards if the platform is certified by POSIX.2 (or whatever name the standard uses these days).
The latest standard for vi (and the rest of the utilities) can be found here:
Microsoft's native UTF-16 really, really needs an editor that easily saves US7ASCII and UTF-8 correctly, both with LF and CR/LF. The native Windows tools are quite poor in getting this right.
This has been addressed in a few realms, primarily shells.
One bash behavior oddity is that, when it is called as /bin/sh, this will work:
$ cat pbasher
#!/bin/sh
alias p=printf
p hello\ world!\\n
$ ./pbasher
hello world!
However, changing the shebang to #!/bin/bash results in this:
$ ./pbasher
./pbasher: line 3: p: command not found
This is because an alias in a script is a POSIX.2 standard, but this historical bash did not allow this.
Forcing POSIX mode enables the alias:
$ cat pbasher
#!/bin/bash
set -o posix
alias p=printf
p hello\ world!\\n
$ ./pbasher
hello world!
In the same way, platforms that care about POSIX.2 compatibility will adjust the behaviors to obtain certification, as bash has done. I saw HP-UX modify ksh88 into sh-posix, and vim also has a VIM_POSIX environment variable that enables a compliant standard mode.