Supporting Rust is inevitable for platforms that want to remain viable for modern software releases

That's the problem. We've long since learned that virtually all systems are useful to attackers, even if they don't permit access to some poor schmuck's bank account or corporate systems, as a step closer to that access, or as part of a bot net. If we're looking at tens of thousands of dollars of compute costs, that's value for hacking. If it's targeted, flash drives are well known to cross firewalls with ease.

> would almost certainly turn off Fil-C for everything except, maybe, sshd.

Penny-wise, pound-foolish. Recompiling everything is going to take more CPU time then you will ever save from most programs. Optimize the hotspots, not everything.

> If Fil-C can be competitive with Rust's performance for the "I want security" people, then C/C++ will continue to dominate the world, because they and Fortran are the only languages that can deliver for the "I want everything to run as fast as it can because my computers cost a lot of money" people.

Again, you linked to some numbers on this. Optimized Rust routinely meets or beats optimized C++, and unoptimized Rust or C++ was 5 or 10 times slower. This idea that C/C++ are the only languages that can deliver is not backed up by the data you provided, and it's not backed up by reality. There are some people who want the equivalent of racing stripes on their computer, but the really cost-conscious know that optimization needs work and timing and $50K in additional hardware costs is cheaper than another programmer.