Since API's aren't standardized and conversion from one product to another likely requires development work and probably impacts your usage in some way even then - the barriers to switching are high enough that the availability of alternative maps still doesn't represent a whole lot of competition.
This argument is symmetric, it implies all other map providers (who are competitors of google maps and of each other) also have no competitors.
Sure, typically yes. It's not necessary - interoperable APIs exist, and for underdogs they're even attractive as a selling point - but market leaders tend not to play by those rules.
A good competitor can make a drop in replacement possible by aligning their api with that of google's
look how good it went for Microsoft's competitors trying to build a Win32-compatible OS....
Look what Linus and Stallman made to replace UNIX.