I tend to agree that separating the design from the problem might make it hard to evaluate. On the other hand, there is something to be said for finer-grained roadmap-like discussions where we discuss future directions and explicitly acknowledge certain problems we would like to work on.
Currently, I think that is the main role of eRFCs. Going forward, perhaps the right balance would be to use the annual roadmap to layout a broad vision, and use eRFCs to flesh out parts of it by choosing specific problems to focus on?