The issue isn't about microservices, its about isolating dependencies between various components of the system. When any part of the system can reach into the user table then you've created a big ball of mud. Even with a monolith, you should have a defined contract between the user service and the rest of the system. That way you can start with a monolith and move to microservices later as needed. You can also change the underlying persistence layer without disrupting the rest of the system. You could for instance start with mongodb for user information while other system components use postgres. If sometime later you think postgres handle json just as well, no problem switch user over to postgres and the rest of the system is unaffected.

You should really build your monolith in a similar fashion to microservices, just leave out the remoting layer.