I would say that we can reimplement the Stream class on top of new composable streams in the future.
I'd love to do it right now but these streams don't exist yet.

I totally support the idea of new streams design, it looks very attractive. The implementation may take a while and require many labor-hours though. 

asyncio is driven by volunteers, mostly by Yuri and I. We constantly don't implement all our *wishes* at the time on next Python feature-freezing but only a subset of them.

Maybe the situation will change in the future but I have no strong confidence that we finish the design of the new streams in 3.9.

Task group is another very required thing. If I choose between groups and steams I probably invest my time into the former.