Each class of ship should maintain a unique role at all stages in the game. Destroyers currently do not, and the problem is exacerbated in the endgame.
Frigates have the mobility to capture strategic locations as well as harass slower ships. When well-built, they don't have problems surviving and contributing in a large endgame battle.
Cruisers have the durability or long range of capital ships, and sometimes have mobility exceeding that of destroyers.
Capitals, with their durability, range and firepower, obviously have a role in endgame fleet encounters.
Which leaves the destroyer in a very weird spot. If you need mobility, you'd use a frigate for less DP. If you need firepower and range, you'd use a cruiser for slightly more DP, but get a whole lot more durability against tough enemies. The only destroyer that brings something completely unique to the table is the Harbinger, with its ability to leave an enemy ship defenceless for a tiny window. So currently, the best use for a destroyer is to bully frigates in the early game, and even that role is heavily contested by the Pirate Falcon.
I'd love to see destroyers carve out a unique role for themselves, being able to contribute to endgame encounters instead of being a combination of a frigate and a cruiser, while in reality being a worse version of both.
As a thought experiment, think about going up against various endgame fleets. Now, think of any destroyers you'd want in your fleet that you would not prefer having either frigates or cruisers filling that role. For me, I come up blank, except for the Harbinger, in select cases.
I realize this opinion is likely controversial, so if I'm missing something, I'd love to hear your thoughts and reasoning!