You're probably right that we should have a subtyping mechanism.  However, my role as SRFI editor is to facilitate, not to guide.  I'm supposed to help people craft good proposals around their own ideas, not push mine.  (Keeping my mouth shut isn't always easy.)  I'm encouraged, though, that we now have generators and transducers, both of which are excellent abstractions for collections.  And we still have streams, too.  We need further abstraction along those lines, of course.  And I'm perfectly happy to have the underlying raw abstractions as well, as we do.

I encourage you to make proposals along these lines, and to encourage others to do so.  We need to get those good ideas out there.