one thing that you missed with your fleet calculations, to the second above poster, is that if the enemy does 800 dmg or so, they kill the assailant, for example, and remove that DPS perminantly from the equation. 800 dmg to a capital ship just mars the shields a bit.
But when you factor in it's cheaper just to replace the lost assailant than even build the capital ship in the first place, the point is moot. Fights aren't won or lost based on 800 damage. It's all about "the number of bullets in the air" when capital ships are basically "heavy heavy cruisers" such as they are in the current iteration.
Another thing you aren't calculating for is the cost of the research of all those ships, the cost of the labs, etc etc etc.
They are talking about late game. if you can't make a Kanrak Assailant in the late game because you haven't researched it your doing something wrong.
However you did forget to include that the capital ship can attack planets.
No I didn't. Check the ships:
Karrastra Destructor - Planet Bombing Damage: 23
with two of them that's 46 damage versus the Kortul Devastator's 45.
Now I didn't include the research for the figate/cruisers because their cost is negligable....you're going to do that research regardless if you pop out a single capital ship or not (other than when you try to get "Go big or go home" nobody builds a capital ship only fleet). Furthermore I'm not including the logistical structures required for either set for the same reasons...you're going to build the requsite warfare research stations either way, so the cost is moot. And since we're talking mid/late game the requirements in the way of labs is the same for both situations.
But this is the issue...most people don't build capital ships because they're not worth it for what you invest.
But let's do include their research since it seems important to you.
Sub total from above: 2610c 395m 345c
Karrastra Destructor Research= 400c 0m 25c
Lasurak Transporter Research= 800c 100m 75c
Kanrak Assailant Research = 400c 0m 25c
__________________________________________________________
Total cost: 3810c 495m 470c
v/s the one capital ship's
Initial Cost: 4200c 400m 275c
It's still cheaper credit wise to research all three typs of ships AND build them versus building the one capital ship.
Sure the crystal costs are almost double (which through smart buying through the black market that's neglegable).
Thus, there is no value in the capital ship.
I challenge you to compare it to building that second 50 fleet capacity's worth of cruisers/frigates versus that second capital ship's cost. I'm proving my point, counter me. 
The point of the thread is that capital ships are so weak they're superflous. I can consistantly take that CHEAPER 50 fleet's worth of frigates/cruisers and beat that capital ship.
What's more, if I don't use the destructors, and instead build 50 fleet's worth of Tansporters/Assailants I won't even lose a ship when fighting that capital ship. That alone speaks droves about the overall weakness of the capital ship. It should be at least the equalivent of the smaller ships provided it's cost is higher. It's logical to me if you match things on the smallest equlivent fleet capacity possible and the capital ship losing, it will only scale as fleets get larger. Gameplay experience has proven that hypothesis to me. Previous builds of the game it wasn't terribly difficult to level up capital ships to the point that they became more useful, right now it takes too long for that initial cost to become recouperated by the capital ship's ability to level. When I build my "luxury" capital ship fleets I don't even waste my time with level 1's...I spend the credits to make them 4 (which takes even MORE credits and research) because otherwise they're not worth the initial purchase.
I'm saying they're not only weak, but they're expensive for what little you get. Thus, nobody builds them.