I would argue Linux ended up with decent concurrency because both Sun and IBM poured huge heaps of dollars into making it so with their contributions, plus huge efforts from other contributors. If someone pours same amount of effort into MongoDB they might succeed with a retrofit/rewrite as well. It's relatively rare though.
That operating system is Linux. It started out very simple and was good enough for many people and then kept evolving. Nowadays the alternatives are mainly footnotes in history.
MongoDB is also simple. Its locking approach does not give wrong answers. Its users are happy. Your assertion that the locking can't evolve is bollocks because you have no clue how the database works. While your statement has many elements of truth for a relational database, it is meaningless for the current generation of schemaless/NoSQL databases. Did you know that MongoDB has auto-sharding and replication, and doesn't need locks for that either?