I think you're right on one level: if your codebase is pervasively, implicitly multithreaded, then there's little value in explicitly marking yield points. But if your codebase is pervasively, implicitly multithreaded, then it's impossible to maintain without locking everywhere (and difficult even then), and combining async with (blocking) locking does not work well.

In a codebase where concurrency is carefully controlled and constrained, an async system that gives you visibility into where the yield points are is very valuable: https://glyph.twistedmatrix.com/2014/02/unyielding.html .