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.
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.