The can just name their new products "MongoDB this" and "MongoBD that".
And 10gen wasn't much of a brand name to begin with. Very few people knew it or remembered what it was, whereas tons have heard of "MongoDB".
Initially I had the same thought as the parent post, but this is a good point. No one ever really heard about 10gen; it was always MongoDB. Their events were always Mongo titled (MongoDB Days) and even their emails are @mongodb.com. In a way it was already their primary brand, might as well make it official.