Plus, there's only so much you can "advance" fighter tech without starting to unbalance the game. Although it may sound fun to have everyone flying 12-gun ships at a cruising speed of 140, the game mechanics would start to break down...
Besides, it's tough to improve on the Perseus. 
Uh, there's a difference between advancing technology and "GTF Keres". The new fighters could just be a bit tougher, a bit faster, and they certainly don't need 12 gun mounts. If you need that many guns, your guns aren't good enough.
As for game mechanics breaking down, BTRL and my own Starforce mod (especially BTRL) demonstrate that they won't, even at speeds much higher than what I envision for a post-Capella fighter force (60-70 for heavy fighters, 70-80 for medium fighters, 90-100 for interceptors). As technology marches on, the tactics and nature of fighter combat may change, but it has been constantly changing ever since its inception in the 1910s.
By specifications the Shivans have fallen far behind in fighter design by FS2.
Except they haven't. The GTVA has absolutely nothing that rivals the Mara, Dragon, Nahema, or Nephilim. Even the Basilisk is a lot faster and more agile than the Herc II. The problem is their weapons are artificially nerfed to do less damage to the player, much like the Prometheus R, which, if it were as utterly awful as it is in game mechanics, would not be in service because it would simply not be adequate enough. Always prioritize flavor text and fluff over game mechanics. Game mechanics are designed to provide an interesting gameplay experience, not to accurately reflect the universe. Everything in FS2's storyline indicates that the Shivans have much better technology than the GTVA, despite Petrarch's initial overconfidence, which the
Ravana quickly corrected by blowing away several warships, including at least one destroyer. Frankly, I think the human race only continues to exist in FreeSpace because the Shivans can't be bothered to try hard enough. Even when they had the Sathanas fleet, their attacks on GTVA assets seemed more about keeping the GTVA away from their juggernauts while they played with the Capella star than any sort of serious assault. They destroyed the
Colossus because it had proven itself a danger to their juggernauts, but otherwise their attacks were mostly just harassment.
AoA is probably not the best indication of post-Capella GTVA military, since you're part of an elite force who by the nature of their operations are supposed to have the best tech available.
That will make it more sensible, so it's not like "hey guys we just renovated our whole capital ship fleet...and nothing else!".
Although I still don't get the Kulas. It's not very fast, it's not very agile, it's not very tough, it doesn't have a large missile capacity, and it doesn't have good afterburners. It is bad at everything and good at nothing. Even if they really wanted to go cheap, they could build Ulysses, or Hercs, or some other fighter that is at least decent at
something.