I liked the first sentence or two of your comment but I quickly stopped following.

Any concurrency model has tightly coupled & specific semantics about scheduling and cancellation: that's what a concurrency model is meant to provide. The semantics of "synchronous code" are just treated as natural by programmers because it's what they've been taught their whole lives, but someone had to invent that too (Edsger Dijkstra & Tony Hoare, specifically).

If all you mean to say is "the async model means whether a function synchronizes with a concurrent process becomes part of its contract", yes that's true, but the idea that this is "eroding modularity" and not "including essential facts about function behavior in its type signature" is an assertion of design principle, not a statement of fact.