It's not always that simple. What if your sync lib has some callbacks, and you want to do something in the callbacks that requires async.

You could argue that this mismatch exists even with rust itself, where you have Drop, which is sync, and you might have to do something in your drop that requires async, like closing a network connection.

You have to pass in some runtime handle that you can use to spawn a task to do what you have to do. This is definitely not simple and beginner friendly. Or even worse - say goodbye to RAII and tell people that they have to explicitly call an async shutdown fn.