I'd actually argue the most powerful benefit the Defiant will give the Terran Federation isn't the ship itself but it's archives because the big thing about Prime Universe Starfleet is their ships aren't just warships but libraries. As we see with Captain Picard, every ship contains a massive library about every subject in the galaxy. In this respect, the most devastating element of the Defiant would be the science-focus of the Constitution-class with medical knowledge, physics, biology, xenobioogy, and machinery. We didn't see it as much as on TNG but there were still many moments where Spock could just look up about anything.Even in a world with replicators I doubt you could start working on building a Constitution class from the replicators the ship came with.
I will say, however, if I'm going to fanboy an explanation, I think the Defiant may actually explain why the Terran Empire has identical technology to the Prime Universe better than just "in an infinite number of worlds that's going to happen" as it's clear the Terran Empire is not pro-science or pro-exploration but is going to seriously hamper itself anywhere but the development of weapons. So much so that a century later, they've made NO advances on the Defiant's technology but have just managed to completely replicate it.
