The number of security vulnerabilities due to PHP's crappiness is two orders of magnitude greater than all of nginx vulnerabilities combined.

Yet PHP is a memory-safe language.

Memory safety won't fix anything by itself, it will just shuffle the shit into some other place.

Now if you're claiming that if you take nginx developers and force them to use Rust they'll somehow start writing better code, then that's a valid point. Although I'm in extreme doubt that it is realistic or even true.