Forgive me if this is a question that has been asked a million times before, but it is one that has always intrigued me. In TNG, I understand the practical reasons for the use of old Miranda and Excelsior class ships - they were models that already existed, so money didn't need to be spent making new ships for the show unless they had it/wanted to make new ships. But I'm curious - from what little I know of maritime history (far less than I should), which informs certain principals in Star Trek by virtue of the "navy in space" vibe, ships usually don't serve a straight century. Thirty, forty, maybe even fifty years, but not a straight one hundred. I'm not sure whether ship classes or designs last that long though. Is there some real life precedent for the long life of the Miranda and Excelsior designs? Could it be perhaps that they aren't even the same class anymore, except in outer shell, but the hull design is kept for expedience's sake? As a counterpoint: the Ambassador and Constellation designs are essentially written off as "old designs" prior to the CGI switch of DS9/Voyager. While I can accept the generally held age of the Constellation design as being comparable to, say, the Constitution refit or the Miranda due to the obvious Constitution style elements of her design, the Ambassador is hardly that old. Production-wise, I don't see that it makes sense to abandon the Ambassador at least as a write-off when the show gets to DS9 and the CGI era and yet somehow the Miranda and Excelsior don't get the same write-off, especially in the face of there being newer ships like the Akira around to replace them when CGI comes around. So I suppose it's a twofold question - is the length of the service of the ship classes realistic, and what might the reasons for dropping certain designs be? Apologies if this is the incorrect section.