> I just don't think that we have sufficient evidence to conclude that using type systems to aid the development of concurrent code actually leads to better concurrent code.

This is like saying we don't have evidence that evolution is real.

Type systems are proof systems. If a type system encodes concurrency safety, and a program is valid under that system, then you can be 100% certain that program doesn't have a race condition bug due to shared state between threads.

If a type system can prove buggy concurrent code is invalid code, then by definition it leads to better valid concurrent code.