It appears that you want to have this argument again

Very well. Kindly remember that we're dealing with the real FS2 universe not your own one where you're allowed to claim that the maxim was a mistake in order to dig yourself out of a hole.
Ahem? When I said other long-range weapons I didn't mean misiles only.
What about long-range AAAf's? Or ultra-long-rane flak? Or jsurt lasers with maxim range?
What? they're not in FS2? Neither are 90% of ships and weapons made so far but I don't recall ever stoping anyone from making them.
Cause they aren't in FS2 which means I can invent counterweapons to cancel them out. This is exactly what I mean by you making up ****. None of that stuff either exists in FS2 or is even hinted at. Look at your long range lasers for instance. There are no lasers in FS2 that don't disappate by around 1km. Yet you're increasing their range threefold. Fine if you're pulling weapons out of your arse so will I. What the **** are any of those weapons going to do when I start having a Maxim Mark II with a range of 9km?
See! Anyone can invent crap to win the argument. Stick to what's in FS2 or you've already lost the battle.
Anyway quite frankly you're wasting your time. I don't think that the battleship you assume to be so useful even logical within the FS2 universe.
1) Between FS1 and FS2 the Orion, Leviathan, Fenris and Typhon were all upgraded to use beam cannons instead of the blob turrets they used to have.
Difference in speed. - None.
Difference in fighter complement - None
Difference in hitpoints (and therefore armour) - None (Tell a lie. The Fenris
improvedSo in other words the GTVA managed to upgrade these ships to use beam cannons at absolutely no cost to the internal or external space used on them. So given that we can see that beam cannons don't take up a vastly larger amount of space than blob turrets the question that has to be asked is why didn't they simply stick beam turrets all over the damn ships?
Well the obvious answer is that the limiting factor is not internal space but something else like Reactor power, heat sink effectiveness or plain and simple cost. Whichever of those reasons is responsible it's not something that a BB will necessarily have a lot more of than a destroyer has. The fighter bay on a destroyer probably doesn't take up a huge amount of reactor power. And it almost certainly costs a shedload of money and resources to fill with fighters and weapons.
2) The Colossus represents the best that the GTVA can do in making a super ship. Took them twenty years to build. Hardly bristling with turrets is it? The Colossus has 63 turrets and is 6km long. It was the pinnicle of GTVA design. Their flagship and yet it had a relatively tiny number. Why? Yet again it looks like the beam cannons are using up a huge amount of whatever resource it is that places a limit on the number of turrets you can have. But that can't be space. Or otherwise they could have covered the ship in blob turrets. Or missile turrets! The Colossus has a pitiful number of those for its size. Even the version mentioned in the cutscene has very few. What is all that space inside the Colossus being used for? Again maybe the limit isn't size. Maybe it's money. Maybe putting 50 extra missile launchers on the Colossus would have quite simply cost too much. Or maybe it's not the weapons themselves that are expensive but the reactors to power them.
3) The Hecate is the GTVA's most recent, most advanced ship. Yet the Hecate is actually a poorer combatant than the Orion. If there is such a large gap in the amount of firepower between a BB and a destroyer why the hell didn't the GTVA build one instead of the Hecate? They'd already got the Deimos. You seriously think no one in the GTVA thought let's make a bigger version of the Deimos? Cause that's all a BB is after all.
4) Look at the Iceni. It was built as the flagship for Admiral Bosch. No expense spared no doubt. What armament does it have? ****loads. The Iceni carries as many BGreens as the Orion does. (Yet another argument that space isn't the limiting factor). Why didn't the GTVA do that with the Deimos then? Again maybe it costs too damn much to put so much expensive weaponary into one ship.
5) How much better armoured would a battleship be? Not much. Certainly not to the degree you're claiming. Firstly armour cost money. So yet again we're pushing up the cost of the battleship with every inch of armour you put on it. Armour costs you speed too so that counts against it but most importantly of all why is there an assumption that the destroyer has weak armour just cause it carrys ships? That's wet navy thinking. The destroyer is designed to go toe to toe with the enemy. At least in the case of the Hatshepsut it is. Same with the Orion.
Why is there an assumption that the extra mass of fighters on a destroyer means it must be lightly armoured but yet the extra mass of guns, reactors and heatsinks on a battleship is ignored? I don't think the difference is going to be anywhere near as large as your assumption. Especially considering that you rejected my theory that beam cannons are small last time. Maybe a battleship might have enough armour to get another salvo in but it's certainly not the monster that shrugs off enemy fire that we're hearing about.
So lets sum up. The BB isn't a viable class. Either it costs too damn much to put all those weapons on it. Or it would melt cause the heatsinks are too close together or there would be no way to power all those weapons and they'd have to take turns using them. Even if you reject that there's no real reason to think that it's any more heavily armoured than a standard destroyer. At best it's a little heavier.
Quite frankly I'm of the opinion that the GTVA considered the idea of battleships but rejected them because they realised that at best they might be able to bolt on a beam cannon or two in return for losing the massive offensive and defensive capabilities of carrying fighters. The FS2 destroyer class
IS a battleship. Close to the best battleship the GTVA can build. Its just that they've bolted on fighter launching capabilities at virtually no cost.