B and C are special guest stars, you don't want to diminish the impact of rewatching the story by having seen ships just like them all over the franchise.
How long a ship's useful lifespan is is going to depend on how fast technology outpaces it and how necessary that new technology is to keep up with demands. You can keep e.g. Warp 7 tech around as long as nobody else can go faster (oversimplifying obviously). But if you come out with a Warp 9 engine and then the Romulans have a Warp 9.5 engine a week later, back to the drawing board. Ideally a ship just won't break down period, but then you get into the efficiency of devoting X more resources into an expected Y increase in lifespan.
Personally I like reliability in cars. I'd rather have a time-tested design with the latest improvements over "the totally redesigned 2024 Whatever" that might take a few years to work all the bugs out of.
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