That thought fills me with dread. Rethink is well engineered and has a great developer experience. Mongo isn't and doesn't.

All these comments are starting to make me feel stupid. I picked mongo for a side project, and it's been really nice so far. I have a hard time imagining how one would use it for a complicated and/or large schema though. I assume that is just my newness to it.

Don't feel bad for not understanding how to make it work for a big project, it just doesn't. You can try make it work for a big project, but at that point you are just implementing database level code in your application.

Better to just use something like PostgreSQL. Contrary to popular belief PostgreSQL is actually a better document store than MongoDB via it's jsonb type, operators and GIST/GIN indexing.

Nope, you have it right. Mongo is a lot of fun to play with out of the box. Once you have nontrivial data models, redundancy and load, not so much.

I'm not entirely to sure if I understand your comment, however, what I mean is;

The company behind MongoDB is a database company so they probably have some sort of affinity and appreciation for a product like RethinkDB, and, they are commercially successful (or at least, it appears to be that way).

Just because, hypothetically, MongoDB could be a custodian for RethinkDB don't mean that all of a sudden RethinkDB becomes MongoDB.

As Paul pointed out, both solutions seem to be two different products covering two different use cases and audiences and from what I can imagine, they might actually complement each other.