Yeah, it makes no sense that a spaceframe that large and complex would be outdated after only 40 years. They'd have made it with future upgrades and refits in mind. Look at 20th century technology like aircraft (4th generation fighters like the F15, F16, and F18 are all designs from the 1970s and early 1980s and are still totally relevant today) and naval ships. You'd think by the 24th century, we'd be even better at technology re-use via upgrades
Actually, I've always felt that the Galaxy class represented the last configuration of previous ship classes such as the New Orleans, Challenger, Springfield, Nebula, Cheyenne, Freedom and Niagara. It happened to be the largest iteration, but nowhere near the newest, and that type of spaceframe was actually on its way out, in favor of more streamlined designs such as the Intrepid, Sovereign, Nova and Prometheus classes. So I didn't really see the Galaxy class lasting all that long, despite what Sternbach & Okuda's tech manual stated.
Also- there's already precedent for it in the Star Trek universe. Klingon designs have lasted for over 100 years. The Excelsior, Oberth, and Miranda class starships are all seen in action 70-80 years after they entered service.
That's just because of Star Trek producers' cheapskateness. If time and money was no option, we would never have seen any of the movie models in TNG. We would have gotten new and updated designs for Starfleet and the Klingons from the get-go.