The current architecture is not designed to be horizontally scalable. You need one big primary database node. That works well if you are willing to optimise a lot, see the one example: stackoverflow. But it requires a lot of efforts and cannot scale that much.

One could probably create a new implementation that scale though.

I'm very new to the project. Do you happened to know if the software architecture is inherently coupled to that physical tech choice (one big postgresql)? I have seen some well architected RoR apps sidestep that limitation, but they seem to be the exception.