Maybe you are right to say defenses are overpriced, yet it is not true that defenses = hp, because, as says Kyro in a post (https://forums.galciv2.com/?forumid=421&aid=139395), "the maximum defense rolls reset after each round", wich is not the case for hp.
However, the defences don't reset if the ship is dead. Given the larger logistics allowance, people can field pretty large fleets, so, depending on hull size, only a small fraction of your fleet will be able to recharge. Most will either be untouched or dead.
However, it's not true that for the ship to survive it's defence has to equal the attack power of the entire fleet. For a high-defence ship, it will generally stop twice it's rating in the appropriate weapon. This is because weapons roll individually while defences roll collectively. So the defence roll will usually beat the attack roll, until the defences become depleted.
Still, it's very expensive to build a ship that will stand up to even a pretty small fleet of equivalent tech. Standing up to the large fleets made possible by logistics is simply not possible at equivalent tech. If it were possible, you'd be invulnerable.
Overall, I think defences are too expensive in the likely situation that they are only used once in a large battle.