IBM has been trending towards death for decades now and it's nowhere near dead.
MongoDB is certainly on the road to death, IMO. As has Oracle DBMS since the 90s.
Most companies tend to make more money as their product's growth stalls out (extracting more money from existing customers).
The fact that Mongo had to create shitty-license underpins huge revenue problems. If you look at the major trends and surveys, the ones targeted at people who actually drive database adoption within companies, MongoDB is sliding YoY for 3 years now.
The place you see Mongo growing is Atlas. Yes, as their competitors can no longer offer MongoDB in their clouds, revenue shifts to MongoDB. That does not mean that use of the database itself is growing.
Find me a bunch of developers and infrastructure people who are EXCITED about running MongoDB. I guarantee you're going to find a 7-10+ year old infrastructure if you do.
That's not the same as starting a new product on MongoDB.
I like to check this site every couple months for stats on DB popularity
DocumentDB is a closed source proprietary database created by Amazon to emulate the MongoDB API. Think Google's Dalvik runtime vs Sun/Oracle's JVM.
This time around we have an open source fork of ES with big backers all contributing and very permissive licensing.
In both cases, Amazon gets to implement AWS-specific upgrades to management to depend heavily on EBS replication rather than application-layer replication. Would it be nice to have that secret sauce that makes Aurora/DocumentDB so nice to use compared to self-hosting or RDS? Of course. Do we have to have it to consider using or contributing to the open source software? No.