Just noticed something that I kind of want to point out.
Pacific is really heavy on anything but actual light vessel ship girls. There are far more battleship ship girls than destroyer ship girls.
So how does it work out then? What are the downsides to a battleship ship girl that you would not want to have them do everything? Why do destroyer ship girls even exist if they have bigger guns?
I think this question is more like, “why do we even have tank destroyers if we have tanks already.”
Lots of reasons. I mentioned rate of fire in another one. Speed is yet another. Supplies and logistics a third. Fairies a fourth. The ability for DD girls to carry a larger assortment of utility equipment a fifth. “Stealth” a sixth (DD girls are harder for the Abyssals to detect).
Furthermore, each shipgirl is unique. They all bring something unique to the table. Even Raleigh. There is essentially some skill that each shipgirl is (more or less) uniquely good at that no other girl can do just as well. Some of these might not be combat-based. Some are.
DD girls hold the largest range of “powers,” ranging from superior reflexes and “bullet-time” speed (Maury) to seafaring brick (Tasha, though to be honest a couple of other DD girls share this trait) to improvisational combat (O’bannon) to magical healing touch (Shaw) to visual calculation & assessment (Mahan) to auto-fairy-air-cover call-in (Edsall) to eidetic recall + projection (DD girl, vol. 3) to mesmerism/enhanced charisma (DD girl, vol. 3) and so on and so forth.
In fact, the one DD girl who doesn’t seem to have a unique niche so far (also in vol. 3) seems to be an oddity. Maybe she really isn’t anyone too exceptional. After all, her historical counterpart does have a nickname of “the destroyerest destroyer,” having done pretty much literally everything a destroyer could ever think to do.
Or maybe she’s got something else, too. You’ll just have to watch and see.