Postgres's extension APIs are pretty good and Rust's interoperability with C makes it quite feasible to extend the database without rewriting it. Some months ago we wrote a logical decoder -- to capture and forward the write-ahead log as JSON -- in Rust and later were able to pull out some helpful bindings for logical decoders, foreign data wrappers and so forth.

The logical decoder: http://pgxn.org/dist/jsoncdc

The bindings: https://github.com/posix4e/rpgffi

The extensibility of Postgres is really profound -- there are so many things you can do, and so much internal functionality you can access, without having to recompile the database. The way extensions are loaded also mitigates the risk of what can be admittedly tricky issues relating to safety. Extensions run in backend processes, not the postmaster.

If rewriting Postgres happens, I hope it would be gradual. You can replace a lot of the core with extensions, and the scope for extension is ever increasing. So say you wrote a storage engine in Rust. Writing a foreign data wrapper for it would give you a way to use it from Postgres for all your tables; but you wouldn't have to write a SQL parser, client manager, permissions system, or any of the rest of that stuff.

Although the C stuff is a little gritty, a lot of what goes on in a database is gritty "because it is" and that low-level control is needed to deliver on tail latency, fault tolerance or simple storage efficiency. For example, Postgres has code to recover from malloc failing.