It's not that [ can be anything (practically speaking it can't, actually, since it's often/usually a built-in that supersedes PATH). It's that [ obeys the rules of the 'language', where it is a program run on its arguments.

I agree that the surface syntax is bad. Unlike other languages with clear Algol influences it never got past thinking writing if backwards to end the block is clever. I'm all for fixing these sorts of things, if you're going to break compatibility with posix anyways.