I'm sure the Federation would field some larger ships in an extended war with the Klingons. I just don't think they'd look exactly
like the ships the Federation produced in peacetime with exploration in mind. Since the war timeline does
have a ship that looks exactly
the same, it must indicate something
about the similarities of the timelines.
At any rate, I still find it hard to believe that, in a prolonged war with the Klingons, Picard, Riker and the others all ended up in the exact same positions on the exact same ship. Again, that their career paths converged so exactly
must say something about the similarities between the timelines.
Perhaps the parallels between the timelines say that the Federation didn't take the war as seriously as it should have in the early stages, so they focused just as much on exploration as they did in the prime timeline, but Starfleet found itself ground down by the relentless Klingon advances until they realized they should have been putting more resources towards the war then had been. By then it was too late....