Other than multi-document transactions, many of the changes to MongoDB since 3.6 are to do with performance and scaling - this is AWS' bread and butter.

Something to consider is how many applications depends on cutting edge database features. Developers often consider a database an object persistence mechanism whose details are to be tidied away. See, for example, the emphasis on database portability in ORMs.

MongoDB's most effective enterprise offering is support, in my opinion. They know their database well, which can make a big difference if your company is heavily reliant on it.

I suspect for many users though, AWS support will be good enough. Presumably, that's the danger to MongoDB.