RethinkDB wasn't proprietary. It was open source, licensed under the AGPL v2, the same license used by MongoDB, which is doing just fine. MongoDB is one of the top five in the DB-Engines rankings.
One of the key business differences between RethinkDB and MongoDB is that the latter keeps some enterprise-grade features/tools closed-source and available under a commercial license, i.e. the "open core" business model.
One of the key business differences between RethinkDB and MongoDB is that the latter keeps some enterprise-grade features/tools closed-source and available under a commercial license, i.e. the "open core" business model.