We know (at least from extracanonical and deuterocanonical sources) that Starfleet launched into a major new shipbuilding effort after Wolf 359, replenishing their depleted fleet and developing new ship classes geared more toward defense and combat. The Defiant
classes were meant to be products of this effort, and presumably all the other new classes introduced in FC and wherever were as well.
Indeed, wasn't there something in the Titan
books about the Luna
class being put on hold because of Starfleet's shift toward a more military-oriented set of priorities?