>nope async streams weren't really possible before without an unnecessary allocation.

You would have to define a few custom types. I have written code that looks something like

  while (! ctx.IsCancellationRequested)
  {
    var data = await source.GetData();  
    await Process(data);  
  }

Where source is an interface that we have defined. This is not much more complex than an async enumerable.

It's definitely possible, and if you save an allocation then that's a step up, but not a game changer for most business process.