Same thing with operetta. If operetta is essentially a synonym for singspiel or comic opera, no, it is not a synonym for comic operathen there is no reason not to apply it retroactively.
So opera/operetta seems more like a cheatsheet to know whether you will get a happy ending or not. no
operetta, as an inclusive term to describe non-opera musical theatre, has spoken dialogue; opera doesn't. within the inclusive term, you can than differentiate between operetta per se (French/Viennese) and singspiele, musicals, etc. (vaudeville probably cannot be included in the term operetta in the first place).
that's why the original Carmen was called Opera Comique, because it had spoken dialogue.