> Also, going all-in on async doesn't seem that helpful on the JVM where the vast majority of existing code (= what makes the JVM valuable) uses synchronous I/O.
I'm not sure that the majority of many-year old backwards compatible code being sync IO has much to do with the value of async. So long as there're ample amounts of async code/libs. I wrote a backend in Scala using an async Postgres lib. I hopped over three different ones before writing my own, all of which are async. For things like network IO in fact, there may be more maintained async code than sync code.
I'm not sure that the majority of many-year old backwards compatible code being sync IO has much to do with the value of async. So long as there're ample amounts of async code/libs. I wrote a backend in Scala using an async Postgres lib. I hopped over three different ones before writing my own, all of which are async. For things like network IO in fact, there may be more maintained async code than sync code.