A startup which I have helped architect back end uses mongodb for everything. before starting the project I have requested the CTO not to use mongo as it was

A startup which I have helped architect back end uses mongodb for everything. before starting the project I have requested the CTO not to use mongo as it was not a right fit.Basically they needed more of a relational stuff.The CTO chose mongo because he was thinking every startup uses it and why not us. Now they are suffering as they need ACID and relational features. They want to rewrite to postgres but they are heavily invested and not easy to go back.