[NotRustAgain]

> but the library it ships with is going to privilege one over the other

This really just seems to me a matter of no one having done it right yet...

Following your example (which I like), I can have both mutable and immutable types in my language. Your point is that the library will favor one or the other, but I think you admit both libraries are possible. Why can't I have twice the library?

Yeah, it might be more work to have a library twice as big, (and again, no one seems to have done it yet) but it's not more work than having two languages with two libraries.

All that to say, you're summarizing what's been done, but you haven't proven that something better couldn't be done.