For my part, I've tended to be satisfied with the build ratios I've seen in many offscreen works, although I admit I consider myself being a "big fleet" person in terms of Starfleet being willing to build 50-100 cruisers or destroyers if the designs work well enough. Generally ships like the destroyers, cruisers and frigates tend to have higher build ratios (let's say 30-60 ships in a given class, optimally) in a lot of tech manuals while more specialized designs like scouts and transport/tugs get moderate builds. Frigate is kind of a weird case in that term having evolved a bit from how older works like FASA would have treated it.

I admit, I kind of like the older concept being sort of a mission cross between a cruiser and a destroyer, able to perform exploration functions but also having the firepower to act as a tactical support as well.
Fully tactical vessels, like dreadnoughts and the Defiant class, do exist as part of the fleet but the build rates are much smaller because they are specialized warships, and the emphasis on more peaceful missions tends to limit the need for such vessels even in eras when tensions with neighboring powers like the Kiingons are high. I've always found it silly to presume that Starfleet could spend the bulk of its existence dealing with such potential threats and never employ a dedicated warship. But I also tend to think that a single dreadnought could, say, operate in place of several destroyers and achieve similar results. Generally most tactical vessels in fan works I've seen, such as Jackill's, don't go much above 20 vessels in a class max (and even that's often a highball estimate given their role). The Excelsior would be kind of the odd duck in this case, if one wanted to keep the FASA designation as a battleship and not a heavy cruiser as other sources have suggested.
I also think the sheer vastness of space, even with Trek levels of technology, is a factor as well. You'd need a lot of ships and infrastructure to continue exploring and building, even assuming that you could do most of that without any neighbor conflicts.

And I don't generally see a problem with a successful design staying operable for 50-100 years or even longer, if it functions well and can be upgraded routinely. The TNG Tech Manual suggests the Galaxy class has a 100-year design expectancy. We already have modern day designs that can last 50+ years, and some of those have stayed in active service for close to a century even fi their operational production period was much smaller (the B-52 being my favorite example - it was only built for a decade but current planes have gone multiples of that in service).
Someone suggested that one reason the Klingons seem to keep so many older vessels around is that they would be considered battle records of a sort, and I think that's a good suggestion with their culture. The Star League of the Battletech franchise developed a "rotational" system of sharing technology, such that the most elite League units (the Royal regiments) got all the best stuff first and then gradually replaced it with newer technology, and then the older stuff was open to the member states. This way the members were able to share the majority of the technological advancements the League was able to create, while the core state of the Terran Hegemony was also able to maintain a good control of how that rotation worked. They didn't generally have to worry about a member state suddenly becoming too powerful.