Say you write a server, one thread, one client at a time. Easy peasy, terrible throughput and performance. Well, making it thread-per-client is trivial! Easy peasy, much better throughput and performance, but terrible at scale.

Now you want to scale, so you need to abandon the thread-per-client, everything-is-synchronous-looking model and start doing async I/O the hard way (perhaps with await, perhaps as callback hell). That's a huge refactor / re-write exercise.

But you'd be surprised how many thread-per-client apps exist. It's so easy to write them...