If you're more interested in horizontal scaling, you may want to look into CockroachDB, which has a Postgres compatible protocol, but still quite different. There are a lot more limitations with CDB over Pg though.
With the changes suggested, I'm not sure it's the best idea from where Postgres is... if might be an opportunity to rewrite bits in Rust, but even then, there is a LOT that can go wrong. The use of shared memory is apparently already in place, and the separate process and inter-process communication isn't the most dangerous part... it's the presumption, variables and other contextual bits that are currently process globals that wouldn't be in the "after" version.
The overall surface is just massive... That doesn't even get into plugin compatibility.
