The biggest argument against it is falling into the "pure technical" trap.
Postgres has a great focus on solving real problems for real people. Losing that focus would be a disaster.
>What's the argument against possible Rust porting of Postgres?
That's putting the cart before the horse. The right question is:
a) What's the argument in favor of porting Postgres to Rust?
and
b) What's the opportunity cost?
Because of the interop ease, it's not hard to imagine a multi-year peicemeal rewrite. Postgres' code has some pretty good idioms that could be nicely encoded in Rust, and a good separation of concerns