Count me in on Andy's side of the bet. The most useful features of graph databases will likely be subsumed into RDBMS just as features from JSON stores and object stores were before them.
For example...One of the hits against RDBMS is that the structure is supposedly "rigid." That's simply not the case in many RDBMS, such as those using column storage. Adding columns in databases like ClickHouse is a trivial metadata operation. This means that many problems that Neo4j solves can be addressed in a more general-purpose RDBMS, because you can add columns easily to track relationships. It's pretty easy to envision other improvements to access methods to make searches more efficient.
I don't mean to undercut in any way the innovation of graph databases. It's just that the relational model is (a) extremely general and (b) can be extended.
Not a fan of graph dbs? Surprised the $325m round for Neo4j didn't make your funding paragraph.
Graph databases will not overtake relational databases in 2030 by marketshare.
Bookmark this comment. Reach out to me in 2030. If I'm wrong, I will replace my official CMU photo with one of me wearing a shirt that says "Graph Databases Are #1". I will use that photo until I retire, get fired, or a former student stabs me.