Good arguments, though only the last one seems like a true objection.

I think, If I were implementing a database from the ground up, I'd indeed make my own query language--SQL does suck--but I would probably strive to accept SQL as an option, with the proviso that it's "just SQL syntax with ThisDB's semantics."

This would let people point their old applications, written under "the database is just an SQL-speaking dumb store for simple CRUD operations" assumptions (e.g. any ActiveRecord app), at the new DB and get somewhat-sensible results out, while still letting them write new applications against the new API.

The alternative is forcing any user with existing services to hack up some sort of application-level back-and-forth replication between their old DB and your new DB during the long-and-possibly-indefinite migration period, rather than just migrating in one fell swoop.