Check out RethinkDB, a competing product which had exactly this happen earlier this year. I don't follow the project closely, but the Linux Foundation picked up

Check out RethinkDB, a competing product which had exactly this happen earlier this year. I don't follow the project closely, but the Linux Foundation picked up stewardship and development appears to be continuing, if at a reduced pace. MongoDB is pretty popular, so while it would certainly be disruptive in the short term for the sponsoring company to go out of business, continued community development is all but assured.