I don't think retrofitting existing languages/ecosystems is necessarily a lost cause. Static enforcement requires rewrites, but runtime enforcement gets you most of the benefit at a much lower cost.
As long as all library code is compiled/run from source, a compiler/runtime can replace system calls with wrappers that check caller-specific permissions, and it can refuse to compile or insert runtime panics if the language's escape hatches would be used. It can be as safe as the language is safe, so long as you're ok with panics when the rules are broken.
It'd take some work to document and distribute capability profiles for libraries that don't care to support it, but a similar effort was proven possible with TypeScript.
I actually started working on a tool like that for fun, at each syscall it would walk back up the stack and check which shared object a function was from and compare that to a policy until it found something explicitly allowed or denied. I don't think it would necessarily be bulletproof enough to trust fully but it was fun to write.
As long as all library code is compiled/run from source, a compiler/runtime can replace system calls with wrappers that check caller-specific permissions, and it can refuse to compile or insert runtime panics if the language's escape hatches would be used. It can be as safe as the language is safe, so long as you're ok with panics when the rules are broken.
It'd take some work to document and distribute capability profiles for libraries that don't care to support it, but a similar effort was proven possible with TypeScript.