Mongo already had "won" the NoSQL battle. So if rethink had made it compatible with mongo's query language, I can imagine that when Mongo was getting all the bad-press many would have jumped their backend to Rethink, without having to worry about modifying their application.
Of course, this is with the benefit of looking back, but the parent's hypothesis has legs.
What are you talking about man? There are tons of successful NoSQL databases with non-JSON query syntax. Neo4J, Redis... should I continue. I never missed this format using ReQL and it would be a bloat to support both in parallel.
Exactly. RethinkDB had basically the same traits as MongoDB, with real-time features as a bonus. It was never really competing with other RDBMS (that support transactions, etc.), it was competing directly with MongoDB.
Of course, this is with the benefit of looking back, but the parent's hypothesis has legs.