> You can end up writing nearly the exact same code twice because one needs to be async to handle an async function argument, even if the real functionality of the wrapper isn't async.

Sorry for the possibly naive question. If I need to call a synchronous function from an async function, why can't I just call await on the async argument?

    def foo(bar: str, baz: int):
      # some synchronous work
      pass
    
    async def other(bar: Awaitable[str]):
      foo(await bar, 0)

Nothing and that’s the problem because even though you can do it, your event loop will block until foo has finished executing, meaning that in this thread no other coroutine will be executed in the meantime (an event loop runs in its own thread. Most of the time there is only the main thread thus a single event loop). This defeats the purpose of concurrent programming.