I'd wager that the vast majority of schema changes are adding new columns or new tables-as-child-relations, so to be fair those are free in a schemaless DB. So the "trivial" case is actually pretty common.

Honestly, I think the difference between schemaless and SQL corresponds very closely to the differences between static and dynamic-typed languages. The schemaless approach provides faster iterations and the ability to simply express things that require elaborate language features in a static-typed language. The difference is that in a DB, you have to support the data created by all your previous versions, so a DB requires far more discipline than the application code.

I can see the appeal, but I'm not sure that it's worth it.