The "Nero changed everything" line is overused to the point of being a crutch. First of all, after destroying the Kelvin, Nero ran off an did fuck all for the next twenty years. Even if the sudden appearance of a Space Octopus tearing apart a starship in ten minutes was a kick in the ass for Starfleet to militarize and start building larger starships, twenty years of no Octopus sightings would have created a sense of complacency and "why are we building these huge ships to combat something which may never show up again?" Hell, five or ten years of Nero being a no-show would have created that kind of mentality. Sure there'd be a hardliner like Admiral Marcus wanting Starfleet prepared for anything, but the most he should be able to accomplish is building something like Vengeance in secret.