>But given that they want to go from Mongo to a graph DB, the fear is that this is someone who is only chasing the next cool technology and not solving an underlying business problem.

To be fair to the teach lead, I do feel like it was the other way around. MongoDB was foisted on us on a new project (we were previously SQL) by a software architect who left soon after. I've never felt that MongoDB was a good fit for what we want to do, but I want to return to SQL.