> which use a cloud managed DB solution (GCP CloudSQL PostgreSQL) it means something like this would never be available

That's up to GCP then. I'm sure OP would be willing to license pgrust to them for an appropriate price.

What's the incentive for third parties to contribute, knowing just one guy stands to benefit if the project gets monetized in such a manner?

Most of the same incentives of typical OSS contribution? It's not like other OSS projects pay out any earnings made with the product evenly across contributors.

Also, from the trajectory of the project it doesn't look like they are significantly deoendent on outside code contributions.