I sorry to say your analogy is flawed by its own strengths
Have you seen the AA/Anti Missile batteries on a Aircraft Carrier? 1000 rounds per second and phalanx guns, even on WW2 ships they had aa batteries.
Aircraft carriers are not sitting ducks, that would be stupid as someone can just suicide a fighter and launch all 4-6 missiles, and there goes billions and billions of dollars.
It's all about who has the better tech, right? Have you seen this?
WWW LinkJust one or two modern torpedoes (not even getting into the "hyper-speed torpedo" category, which is feasable), and a delivery system that's stealthy enough... that's all it takes.
Yes I have seen that actually

Torpedo range is a great distance. Just because its in range, doesn't mean the target will sit there and wait for it, (sub has to compensate for angle on bow, ship speed/course, oceanography, etc etc, many of these variables can change as soon as the torpedoes are launched, or during payload delivery..not to mention detection methods.
However going by this logic path we will undoubtedly come to nuclear weapons, MRLS Artillery, ballistic/cruise missiles to take out carriers. I never said they were impregnable, however used in relation to the fleet model in GalCiv2, (no missiles, no submarines/cloaked units)...
In the context of THIS game, there is nothing similar to a Submarine, ICBM, or a cloaked ship, thus rendering this example invalid in the context of terror stars.
Anyways, I don't want to derail this topic further, but you do have many points in your post that I agree with, I just objected with that one element/implication.

And to end and come full circle ...your point... "Just one or two modern torpedoes..."
I am not asking for the Terrorstar to have 100 defense in all categories, I am not asking for a hi tech defense tank. I and seemingly what others are clamoring for, is some lower tech stuff, (say 5/5/5 or 10/10/10 just for example) in attack defense.
Heck you can even make it that the only armour a terrorstar can have is the Top level Starbase defense when you've researched the appropriate defense technology, ( overlord Armour, Electromagnetis Surge, Invulnerability field... [the 16 ones]) That would stop thins from being overpowered defensively, but it would still need a comparable low weapon.