Godwinson
"Well, I've got nothing."
What I believe Oneiros is saying, is that any new designs need to fill a specific role and do so better than its predecessor, to keep the number of different designs in place from ballooning massively. Not that this is some longstanding policy, but rather, a reflection of the way that Starfleet's relationship with the Federation Council is evolving in the new century. Old designs are grandfathered in, and refits aren't a concern for it.