The standard library is different to the language itself: the language certainly doesn't disallow allocators/OOM handling, it's just the design of std. Furthermore, the standard library is layered: there's `std` with various OS-required routines (IO, etc.) and `core` that is the core stuff that doesn't require any of that. Operating systems/embedded applications can generally use `core`. However, even that is better than C/C++, where one usually has a from-scratch "standard" library (which is possible to do with Rust too): not being able to use the compiler-bundled `std` doesn't seem like a point against Rust.

Many user-space applications don't do heavy network work, which is where concurrent IO is most necessary. E.g. games or a scientific simulation, even web-browsers don't need of concurrent IO (they're not trying to juggle thousands of connections).