Moving fast and breaking things in the database world is not a good thing. That said... I highly doubt collection level locking will take too long.
I don't see anyone nipping at their heels. They have some competition in the key/value space, but there are a few different KV paradigms.
MongoDB is a complete replacement for most RDBMS apps and they are way ahead of the pack.
David may use MongoDB at really high scale, but since when is that a reason to call someone an expert.