Executing unsigned code on iOS is generally not feasible, which means that this would limit the set of programs you could run to a small prespecified (and signed) set of programs. (These apps exist already on the App Store; here's one: https://blink.sh). iSH can run user-created programs by emulating them. (Plus, with emulation we can run programs without modifying them because we can trap syscalls and such.) It would be nice if we could JIT compile code (since the interpreter is a bit slow compared to native code) or utilize hypervisor facilities, but that doesn't appear to be an option for now.