Star Trek First Contact - Why only one Defiant?

The following article is an edited lengthy comment I wrote on YouTube in a thread discussing the Anti-Borg capabilities of Star Trek's Defiant class starship, specifically in regards to the Battle of Sector 001 in the movie "Star Trek: First Contact".

Earlier in the thread it was questioned why Starfleet did not field any additional Defiant class ships besides the USS Defiant in that battle. This comment focused on answering that question specifically.

I do know that history of the Defiant that you discussed - but no matter which way the discussion is sliced (when we ask why more Defiant class ships weren't seen at the Battle of Sector 001) it's important to remember the dates. Since the Battle of Sector 001 is definitely set in 2373, we have a somewhat clear picture to evaluate. We have two named ships confirmed to have been launched prior to the battle: the Defiant herself and
the Valiant. The Valiant was launched in 2372 but was specifically designated as a Cadet training ship which was sent on a long term tour across Federation Space. The Valiant is essentially disqualified for battle consideration for those two reasons - remember the only reason the Valiant saw combat for so long during the Dominion War later that year was because the ship was literally trapped behind enemy lines and had little choice in the matter.

As for unnamed ships we see at least 2 Defiant class ships on screen alongside the Defiant during fleet actions with the Second Fleet at the end of "Call to Arms". It is possible that there are additional ships by this point but we don't have confirmation. As of 2373 there are only 4 confirmed Defiant class starships visible on screen. 

Now, the USS Defiant herself arrived at DS9 early in 2371 and was at that point considered a highly experimental failure. Yes, Miles O'Brien did miracles with that ship but we cannot assume that Starfleet Command took an immediate liking to the completed ship. A design that had been so heavily plagued with problems that it was mothballed for several years would not be immediately welcomed with open arms simply because one engineer seemed to have fixed what an entire team of engineers at Utopia Planitia couldn't. The Defiant would still require considerable field shakedown testing before mass production could begin. A Cadet training ship that would never be pushed to it's limits, launched in 2372, was a perfect testing solution - testing both the ship and the cadets at the same time. Since both ships performed their tests exceptionally, thanks to the solutions offered by Miles O'Brien and the DS9 crew - mass production of this class would finally become a reality, likely beginning in late 2372 or early 2373. 

Since we do not know the exact dates of some of DS9's episodes we can only estimate that the Battle of Sector 001 took place shortly prior to "In Purgatory's Shadow" as a quick reference is made to a recent Borg attack on the Federation. This easily places the Borg attack in very early 2373 while "Call to Arms" (where we see the other unnamed Defiant class ships) doesn't take place till near the very end of 2373, several months later. 

This gives us a couple of possible scenarios to work with: namely that the two Defiant class ships we see in "Call to Arms" were still under construction at the time of the Battle of Sector 001; or that like the USS Valiant they were already launched by the date of the battle but were given distant assignments and could return in time for the battle. One possibility is that the ships seen in "Call to Arms" were assigned to the simultaneous Federation-Klingon War and retrieving them for the Battle of Sector 001 would have neglected their fleet assignments in that conflict, a decision that could have been costly. All other Defiant class ships (including the Sao Paulo) are seen after "Call to Arms" and upwards of a year or more after the Battle of Sector 001. This also includes the 4 Defiant class ships seen in Star Trek: Voyager, episodes "Message in a Bottle" and "Endgame", taking place in 2374 and 2378 respectively. 

While there is still some window for more Defiant class ships to be operational prior to the start of 2373, the window is still pretty narrow when you consider all the dates. It would have been logistically extremely difficult for Starfleet to field a large fleet of Defiant class ships within a timeline of less than 2 years total for testing, shakedown, refinement, and new construction. Remember, this is the same Starfleet that spent months refitting just 1 Excelsior class ship and the same Starfleet that only appeared to have 2 Sovereign class ships after at least one year of shakedown time. Rapid mass construction might seem good on the surface but it isn't something that can happen over night. However, I do agree with you that had the Borg attacked after the Dominion War there would have been a considerable number of Defiant ships present in that battle, with at least 2 seen within minutes of Starfleet's detection of a Borg transwarp signature in Voyager's "Endgame" (though interestingly, the bulk of that fleet was Galaxy Class).