Re: Why so many ship classes?....it doesn't seem realistic.
The U.S. navy currently has 22 active classes of ships.
Don't get me wrong, that's a good point ...
World wide there are over 3,000 active naval vessels, which include ....
20 aircrat carriers of 10 different classes.
169 destroyers of 32 different classes.
422 Frigates of 49 different classes.
... but at the same time they aren't building four or five different types of attack subs ...
Of the 398 attack subs in active service, 59 subs of 4 classes came from America (43 Los Angeles class, 2 Seawolf class, 6 Trafalgar class, 8 Virginia class).
5 different active service attack sub classes were built in Germany, 6 in Russia.
It occurs to me that all
the Nebula class ships might be precursors of the Galaxy class, the Galaxys are a follow on class, with the Nebula construction being discontinued prior to the construct of the first Galaxy.
The Brits might get mad at having one of their classes included with the Americans (Trafalgar).
What was worse though in the Cold War, when pretty much the Cruisers, the destroyers, and the Frigates were all doing the same thing (Anti-Submarine Duties), except for a few large WWII cruisers that were converted to flagships. Even the Soviets decided to build the largest warship (outside carriers) devoted to ASW (Kirov's).
In re Starfleet, we just haven't seen them do too much on-screen other than failed explorers, guest star character transports, and cannon fodder in terms of non-hero ships.
"As my sweet mother always said, 'son, if one hostage is good, two are better, and three, well, that's just good business!'"