I don't think 20-30K destroyers/cruisers are a bad idea. The way Trump envisions it as ""Trump class" might be a bad idea, but not a ultra large ship.
Coming from my earlier post, as I saying, in this modern age dominated by drones and missiles, stealth is not going to work for ships. So, what is the best way to survive mass saturation attacks from Drones and Missiles? It is to build such a large scale defense that it can rebuff mass attacks.
Load the ship with ultra powerful radar, multiple types of air and missile defense. Instead of current approach of VLS based air/missile defense, large cruisers should have every kind of air/missile/drone defense possible. So, multiple microwave arrays, multiple lasers, 200+ VLS, multiple CRAMs with big loadout of missiles.