MongoDB had a ton of momentum a few years ago but I think the argument being made is that it lost most of it due to the growing perception that it had fundamental design defects. Just anecdotally, a large number of companies I deal with had MongoDB at one time but I can't think of a one that uses it today. It was not replaced with a single thing but it was replaced for a variety of reasons.

As a long time observer of the database market with no skin in this particular game, in terms of regaining market momentum it is difficult to get past the reputation of being technically defective once that perception is broadly in place. To make matters worse, needing to completely replace the storage engine with a proper design will be viewed by some as an admission of that rather than an upgrade per se. It may be too little, too late for its long term prospects. By contrast, PostgreSQL is doing great right now; just about every company I talk to is using it somewhere.

Putting on my database engine designer hat, MongoDB did need to start from scratch with that storage engine; the original one was a naive design with a lot of problems. The sharding/replication design is still quite dodgy, which will hinder practical scalability. A new storage engine is a start but not enough if MongoDB wants to regain their momentum.