But you see, that was 14 years ago. Even George W. Bush wasn't stupid enough to take years to figure out that America was in for his "War on Terror" for the long haul.
You're assuming they didn't think it would be a "long war" to begin with. We know almost nothing from before the 14 Year War at all, and very little from the early years. The only time we get any indication about the progress of the war is when talking about the production of the Levithan, and it was restarted after only a short period.
To divert from canon, my version of the 14 Year War has a slow build up as each side gauges the other, then a burst of warfare that resembles WWII, then things settle back down into a long stalemate that we see in FS1. It's no less accurate than any other version, I'm afraid, but allows for some of the canon events we see. If we take into account the newly emerging technology that we see in FS1, we might actually see a renewed burst of conflict after a long lull, but I don't know for sure.
I like to think that the Thoth had already been in development for months if not years, and that the Vasudans licensed the Thoth design to the Terrans, who made their own modifications, and bam, the Ulysses was born, not as a clean-sheet design but as a Terranized Thoth. The idea that the Thoth was probably already in the works can also be substantiated by its higher hull strength than the Ulysses. The Thoth with its shields shorn away fits right into the pre-Ross 128 Vasudan fighter lineup as an advanced space superiority fighter.
This would make sense, but at the same time we are told the development was congruent with the Ulysses, not that it was copied out right. The designs of the two ships, on the outside, is so radically different that what elements came from the Thoth are sure to be internal (engines, electronics, etc). So the rest of the ship had to be designed post cease-fire, and then it was deployed very quickly, making its production still a insane.
The Ursa is somewhat harder to justify, with its relatively low hull strength (no one would want to fly that brick into unshielded combat with 550 hitpoints), but I'm sure Demon- and Lilith-class vessels would have scared the pants off the GTA and provided sufficient justification for building a new bomber class. You could even go so far as to say they were already working on it and sacrificed some armor for extra reactor capacity or payload in their scramble to get it finished and out the door. Remember that the Typhon was of recent development (2333) and considered an extremely serious threat by the GTA (so much so that I think in actuality it had much heavier armament than mere blob turrets) that they would have had incentive to build counters against it immediately. A squadron of Ursas armed with Harbingers is quite effective at taking down a Typhon. The same unit fielding old Amun-like bombers with bombs worth only 1000 damage or so...not so much.
At any rate, the Ursa is such a quantum leap over the Amun and whatever the Terrans had as an equivalent that I think had the Shivans not showed up, it would have been delayed until 2336 or even later if it were already in development in January 2335. I chose for it not to appear in my Twist of Fate campaign until the second chapter for this reason.
If the Ursa had been in development before the Shivans arrived, and then they rushed the design out the door to combat them, shouldn't a proper version have shown up after the war? Perhaps the advent of shields made the improvements unnecessary, but I would still think they wouldn't have kept a gimped version of their premiere heavy bomber in service for 32+ years. I think the Ursa was specifically designed for going after the Lucifer with Harbingers (the fact that the project for developing both was called Project Ursa supports this). With shields already in production at that point, the Ursa was probably developed with these things in mind.
It's unlikely the Ursa was designed to take on a Typhon, because the Tsunami was probably being developed for that. Harbingers were reserved for planetary bombardment, so the weapon itself existed, but they had to pair it with a decent engine, something that likely came from the Vasudans as indicated from the Interceptor missile introduction:
Our communications with Vasudan technicians have yielded the Interceptor missile. By combining the Vasudan-designed engine with a Terran warhead, we were able to produce the most powerful anti-fighter weapon yet.
I think the Reference Bible says the Vasudans are better engine builders than the Terrans as well (not in a rush to look that up). The point is if the Terrans could have used the Harbinger as an anti-captial ship weapon, they would have, but they didn't, so it seems they could not, at least not without Vasudan help.
Also, I would disagree with the last bit. I tend to see the 14-year war as a 14-year arms race. Even when they were too exhausted to wage frequent battles, they were probably pouring vast resources into R&D much like the US and Soviets--even after Vietnam when they more or less stopped directly interfering with each other's proxy states, they were still racing each other to build more powerful hardware. To me, the new models must have rolled out so quickly because the Terrans already had most of them in the pipeline and the Shivans picked a really bad time to invade. After all, they did seem to be gaining the upper hand against the Vasudans, what with continually banging on Vasuda's front door in Antares.
Operation Thresher failed to produce the results they wanted, and while the Vasudans were all but driven from Antares when the Shivans showed up, they weren't defeated by any stretch. It's probably likely that Antares changed hands multiple times during the war, and this was just another phase of that.
While I do see an arms race between the Terrans and Vasudans, such races are better run when you're NOT at war. WWII saw many amazing developments, but on the front lines, they used what worked and what they could crank out in crazy numbers. After the inital "feeling out" stages of the 14 Year War, it probably came down to just that, what worked and what they could produce in high numbers. So you have millions of ML-16s, thousands of Apollos, even more Anubis fighters. Quantity has a quality all its own.
Which meant crap to the Shivans when they even more, and their scouts were more than a match for the top of the line fighters in the field. Did they have some of these units in the pipeline? Probably, but they had to develop specialized weapons, the Ulysses, the Ursa and the Harbinger. It was a tech revolution, and it changed HOW the Terrans and Vasudans fought wars, something we get to see in FS2 against the NTF.