Fearless concurrency, is only fearless for a very fine grained version of it.

In memory resources shared among threads.

Turns out threads also may share resources like out-of-process files, memory mapped regions, shared memory, databases, distributed transactions,.... where the Send and Sync traits is of no help.

Also you happen to forget Haskell has a Software Transactional Memory, and Swift also has similar protocols since version 6, and effects are a thing in OCaml, Scala, Koka, while languages like Dafny, Koka and Idris also provide similar capabilities via proofs.

That's why everybody who cares about OS safety should invest into capability-based operating systems. They take the concept of Send and Sync and implement at the runtime of the entire computer.