I think I almost completely disagree with you. The product of MongoDB itself offered less than zero added value when it first launched through a lack of data integrity. The $300MM put into it was put into marketing. Mongo tried to squeeze Amazon and users, which is actually great. They are trying to walk the walk now, and what they're going to realize is that they can't. Amazon will out-execute them and hopefully put DocumentDB on stronger foundations than MongoDB's original creators ever could.
Judging on your other comments, it doesn't look like you're familiar with MongoDB's historical shoddiness. I don't agree with you on practice in this case or any case. If MongoDB's company can't out-execute, then they should have never made their product open source to begin with (and in that case it probably would never have gotten as popular as it did). That's the bargain you have to make to try and monetize open source software -- you don't just get a business for free.
But beyond that, I don't agree with you on principle in this case, because I have nothing but disdain and scorn for MongoDB. Both MySQL and Postgres have fine support for unstructured data now, and provide the entire relational toolchest as well. That's not even getting into other multi-paradigm databases like CockroachDB or ArangoDB. I struggle to think of a use case that it is more appropriate for than another option; and indeed, every time I have had to deal with it in production, I have never thought that it was anything but a poor decision.