I am going to recommend sticking to the status quo, i.e. Andrew's improvements to asyncio.Stream should stay. The rest of this message is an elaboration.

The new perfect composable Streams design is just that -- a design. Many things could go wrong in the implementation. Once it exists as a 3rd party add-on and users agree it's better we *may* decide to deprecate asyncio.Stream. Or not -- there are many examples in the stdlib of modules for which a better 3rd party alternative exists but which are nevertheless useful and not deprecated.

Much of the asyncio universe already exists outside the stdlib -- the perfect composable Streams API should probably never be moved into the stdlib (unless it's so perfect that it never needs to change :-).

Andrew's improvements help current users of asyncio.Stream. If those users eventually want to migrate to the perfect composable Streams API, once it's available, fine. But I don't think we're helping them by not giving them improvements that have already been implemented (and which everyone here agrees are improvements!) right now.

Users of the current asyncio.Stream have a choice -- they can adopt the improvements in 3.8, or they can wait for the perfect design to be implemented. Everybody's constraints are different. Let's not pretend we already know what they should choose.

If in the future we end up changing our mind, that's okay. It's happened before, and we've lived.