There's been diversity in starship design all the way back to STIII.
Not often. I don't play STO so I don't know if all the ships posted above are contemporaries, but Trek ships are usually kitbashes of the Enterprise (Reliant, Nebula, Constellation, New Orleans, Cheyenne, Apollo, etc) or other ships (Sydney, Yeager, Centaur), more than entirely different.
...they're also reuses that are the wrong scale (Klingon Bird-of-Prey, Curry, Jem'Hadar battleship, Mirror Negh'var), or reuses of ships long after they should be around (Klingon battlecruiser, Miranda)...
But it's only more recently that you have something like the Discovery, Shenzhou, Enterprise all having different esthetics. Even so, the Enterprise looks Discoverified vs how it looked in The Cage/TOS. That really something, too, given that when new ships are introduced they tend to all look of a certain era. The Wolf 359 ships look D-ish, the First Contact ships (Akira, Sabre, Norway) look angular and of an era...even in STIII the Excelsior and Grissom look similar.
Usually it's only when they introduce a hero ship that they come up with something different – Constitution, Refit, Galaxy, Intrepid, Defiant, Sovereign, Akir-uh...NX, Discovery, Protostar.
All that said, maybe we're dodging a bullet that they don't offer more diversity of esthetics. Half the time I don't buy the timeframes or lineages of the designs as it is. We're supposed to nod and look at the pretty pictures than buy the realism of some of this stuff. And many fans do.