This would be one those places where a language like Rust would be helpful. In C/C++ with undefined behavior and crashes, process isolation makes a lot of sense to limit the blast radius. Rust borrow checker gives you at compile time a lot of the safety that you would rely on process isolation for.
Yes, but note that the blast radius of a PostgreSQL process crash is already "the whole system reboots", so there are not a lot of differences between process- and thread-based PostgreSQL written in C.
Rewriting in Rust would be interesting, but it would also probably be too invasive to make it worthwile at all - all code in PostgreSQL is C, while not all code in PostgreSQL interacts with the intrinsics of processes vs threads. Any rewrite to Rust would likely take several times more effort than a port to threads.
