I agree. I think of address space context switching overhead as the performance price we pay for not being able to run all our programs in a single address space, which we could safely do if we knew all the programs were emitted by a trusted compiler that disallows unsafe memory access. Imagine if system calls were just ordinary functions that can be called with no more than the normal function call overhead? What if they could even be inlined?

Obviously there's a lot of little details you'd have to work out. Like how to make such a trusted compiler in the first place, and how to sandbox unsafe code and legacy applications that were compiled by an untrusted compiler.

If this seems like it's far-fetched or too much work, consider what lengths high-performance hardware devices like HPC network interfaces go to avoid system calls at all costs, to the point where applications talk to the hardware directly. Is that really a sustainable practice long term? And how can anyone audit the security of such hardware devices?

You already see this in PaaS and serverless for managed runtimes, I don't care if my Java and .NET code runs on bare metal, micro-kernel, unikernel, or whatever.

Rust will not replace anything because it’s impossible to write code in it. Everything you write is a syntax error that requires an exobrain to figure out.