We actually considered it at Percona with few other companies in the space and there were not enough interest, unlike in Elastic case.

Keeping reasonably up to date in Tech with 30B+ company by reimplementing key features from scratch is daunting task.

MangoDB approach focuses on PostgreSQL (and PostgreSQL Compatible) databases future innovation which makes long term progress much more feasible.

With DocumentDB and many other partially MongoDB compatible software getting traction I think we will see subset of MongoDB functionality to emerge as de-facto standard in this space