Full Compatibility is unlikely to matter. I think MongoDB figured a great pattern about how to conveniently work with Document Stores from programming languages, and this approach, though not a standard is becoming akin to SQL for Relational Databases. And same as with SQL we will see some features being more important than others and full compatibility basically irrelevant.
I think AWS had a chance to make a greater impact in the ecosystem by opensourcing DocumentDB top level to let people run it with standalone PostgreSQL, yet as they did not do that it is not surprise solutions like FerretDB are popping up to provide MongoDB experience without pesky SSPL limitations.
Biting the hand that feeds IT
