Yes, but. Suppose you build a thread-per-client service before you realized how much you'd have to scale it. Now you can throw more money at hardware, or... much more money at a rewrite. Writing a CPS version to begin with would have been prohibitive (unless you -or your programmers- are very good at that), but writing an async/await version to begin with would not have been much more expensive than a thread-per-client one, if at all -- that's because async/await is intended to look and feel like thread-per-client while not being that.

One lesson I've learned is: a) make a library from the get-go, b) make it async/evented from the get-go. This will save you a lot of trouble down the line.