I don't think this should be done until the Config revamp is finished. I'm not the first to say that either, according to this ;)
Topic on Talk:Requests for comment/Extension management with Composer
What has the "Config revamp" to do with the this RFC? (the link you are referring to as source doesn't contain any information relevant to this RFC).
I think the Extension manager should be merged with the configuration panel of the extension. Dunno.
The wikitech-l archives got broken a while back, the email I'm referring to is .
It might indeed be nice to have a UI that has stuff for both tasks. However, this RFC is not about the UI of extension management at all. I see not reason why this RFC should be blocked on it.