I think the real problem is that SQL text is the only interface to most relational databases. There is a SQL standard, of course, but it's difficult to get vendors to implement the standard, because SQL is the user interface to the database.

Instead, perhaps a second format should be standardized, which is machine readable/writable, that more or less represents relational algebra + whatever extra features SQL supports. It can be clunky and verbose, as long as it's straight-forward and easily composable. It would effectively be like a compiler intermediate representation.

Once you have something like that, you can have as many front-ends as you want with whatever syntax you prefer.

Of course, getting all of the RDBMS vendors to agree to it is still a problem, and they'll probably all still include their own vendor-specific extensions and differences, because they want to lock you in to their system.

But at least maybe the open source ones could agree, which would still be quite beneficial.