OTOH, out of order/speculative execution only amounts to information disclosure. And general purpose OS's (without mandatory access control or multilevel security, which are of mere academic interest) were never designed to protect against that.

A far greater problem is that until very recently, practical memory safety required the use of inefficient GC. Even a largely memory-safe language like Rust actually requires runtime memory protection unless stack depth requirements can be fully determined at compile time (which they generally can't, especially if separately-provided program modules are involved).