How do you come to that conclusion? The Valiant was full of cadets when war broke out, so evidently it had been built. Presumably having seen the success of the Defiant against the Jem'Hadar, where the Odyssey and a fleet of Romulan and Cardassian warships had failed, Starfleet decided to reactivate the programme.
Yet the registry of the
Valiant is mere five "points" higher than that of the prototype vessel, either speaking against the idea of 1000 NCC/yr or then suggesting that these two 74000-range ships date back to the same year the
Voyager does. What this "dating back" really means is unclear: perhaps all three ships were to be completed in the same year, but the
Voyager was delayed by three years over the
Defiant. Or perhaps all the three were ordered in the same year, and it was expectedly quicker to build
Defiants than
Intrepids.
In either case, the
Sao Paulo is only a bit over 1000 "points" higher than the prototype
Defiant, even though apparently built almost a decade later. Perhaps a case for
Defiants actually being extremely difficult and slow to build, so that several keels were laid for the original anti-Borg mission in the late 2360s and given registries, but completing them to operational starships after "The Search" took several years?
Sometimes it's argued that the
Defiant would be mass-built for the war. But she's not
that small, and there's no indication she'd be any easier or faster to produce than an
Excelsior or an
Akira. She might require fewer resources in terms of sheer mass or volume, but those might be more exotic and expensive. Quite possibly she could be a silver bullet type, only built if the need were pressing and an obvious tactical use existed.
Timo Saloniemi