I disagree that async/await is purely about avoiding overhead of kernel threads. Kernel threads are actually not that expensive these days. You can have a server with 10,000 threads, no problem.

The problem is synchronization becomes extremely hard to reason about. With event loop concurrency, each continuation (callback) becomes effectively a transaction, in which you don't need to worry about anything else modifying your state out from under you. That legitimately makes a lot of things easier.

The Cloudflare Workers runtime actually does both: There's a separate thread for each connection, but within each thread there's an event loop to handle all the concurrent stuff relating to that one connection. This works well because connections rarely need to interact with each other's state, but they need to mess with their own state constantly.

(Actually we have now gone further and stacked a custom green-threading implementation on top of this, but that's really a separate story and only a small incremental optimization.)