Mostly because an RDBMS will ask you to explain, explicitly, in complete detail, how you propose to consolidate the entire schema and existing data with the new

Mostly because an RDBMS will ask you to explain, explicitly, in complete detail, how you propose to consolidate the entire schema and existing data with the new changes without introducing paradoxes or inconsistencies. Mongo won't. It will take you at your word. Even if that word is wrong.