I view each SRFI as a separate project, and one that users (SRFI users and Scheme implementation authors and maintainers) are likely to want to use independently of other SRFIs.  Making pan-SRFI changes is rare, but making changes to individual SRFIs is common.  Letting users work with SRFIs separately is generally easier and less intimidating, and makes it easier to disentangle changes in one from changes in another.