> I feel if OCaml had got its act together around about 2010 with multicore and a few other annoyances[1] it could have been Rust

That's about the time-frame where I got into OCaml so I followed this up close.

The biggest hindrance in my opinion is/was boxed types.

Too much overhead for low level stuff, although there was a guy from oxbridge doing GL stuff with it.

That's still the biggest hindrance in my opinion, at least for my use cases. OxCaml, Jane Street's fork, has some features for unboxed types and stack-based allocation of boxed types; hopefully it goes upstream.