I think MongoDB is a closed book. They are the present day Oracle.
If you look back at relational database growth and competition, Oracle by no means stood alone. They had a ton of competition including IBM. And in the late 70s and early 80s IBM was a formidable competitor. But, Oracle had a blistering development schedule, even though the product tended to be buggy, it still improved faster than other offerings. Oracle was the database manager that everything else was compared to.
Although “open source” was not really a thing at the time, I doubt it would have made much difference. Most enterprises are just not going to scrimp on a DBMS. A DBMS is known as middleware, no end user directly interacts with a database, software talking to software is the way a database is communicated with. Companies hate problems with middleware because they can do nothing but report it to the vendor and wait for the fix.
So DocumentDB or MongoDB? Put yourself in the position of the IT manager who has to make that decision. I can get this product that only runs on AWS which emulates about a third of the functionality of the premier product from about a year ago. Or, for not much cost difference I can get the premier product which will run in my shop as well as any other cloud and which I know will continue to be enhanced and supported, and actually is the only offering robust enough to meet our requirements.
I could be wrong, but IMO MongoDB is a very high confidence investment. But then, so is Nutanix.