https://github.com/mongodb/mongo/blob/6a73b990746e1287ce6bad99d4cec263ed420072/src/mongo/db/startup_warnings_mongod.cpp#L336-L343

This seems especially wrong with the adaptive executor where you would want substantially more FDs than threads. It also seems wrong for mongos where each incoming client thread may have many outstanding requests for scatter-gather queries. Even for mongod, you may have many data files but not expect many client threads. Is there any case where this warning makes sense?