By that standard every language needs to die. Or to put it another way, how long should languages be permitted to survive as we figure out better ways of doing things?
Yes, I get to use mature libraries for everything without having to spend development time redoing them, and can focus on business code instead.
Imagine you updated your pom file and bunch of stuff stopped working. If it's easy enough to fix you do it, if not you might just revert pom changes and stay with older version of the language, module or whatever. That wouldn't happen with versioned specification languages.
> I see no particular advantage in specifying the language version in each and every file
You obviously stuck with this mindset and not thinking about it hard enough.
In many languages there is already a mechanism for specifying that kind of info, like pragmas or CDATA in SGML descended markup languages. And it's not hard to introduce either way. Javascript borrowed 'use strict' from Perl 5, for example.
In the end you write something in a language. What language you use to write that isn't defined in some external file, it's an inherent property of the text. You can't write a note in Ye Olde English and expect it to be understood by modern people without issues.