I'm left wondering if the class namesakes of Nova, Danube, Intrepid and California didn't all enter service roughly at the same time as the Galaxy now.
Well, lets take this angle, in the 2270s to 2311 there was a cold war with the Klingons till 2293, then another cold war with the Romulans till 2311, so it makes a bit of sense the Federation would do a Reagan style build up of forces during that time for the just in case of war. so you now have a cheap ship like the Miranda's being mass produced, and by 2285 they have the excelsior, so you have a run of 25 years for Excelsior classes being pumped out, so in that vain, you have a glut of mirandas and Excellsiors, and when 2311 hits.. you have no enemy, like after the soviet union fell, you crunched the military, so any older classes like the Constitution would be decommissioned, along with a whole slew of other classes. as for new build you still have the ambassador and others but probably not produced in mass quantities since you have a glut of ships. so you have a period of time till the borg show up where its quality than quantity with the galaxy class and its ancelleory classes. then you have a building spree with Sabres, akiras,steamrunners etc. and you still have a glut of Excelsiors and mirandas that were probably mothballed or some planetary or patrol fleet that were re tasked with fleets.