Yeah but also remember that the Cruisers are refitted Great War relics really. How many Cruisers were built after the Great War? And how many survived the Great War in the first place?? And if you look at the number of ships which actually appear in the game, then the Deimos is almost on par with the Fenris and Leviathan. It's certainly not a 3:1 ratio by any means.
I know the Deimos is much larger, more expensive and so forth, but if its more cost effective than the older Cruisers, I can see it replacing it very easily. The Fenris particularly is very outdated. The Leviathan I can see having some longevity as the current beam armament makes it an excellent escort.
Who knows, maybe the GTVA will rethink its policies and simply make the Corvette class a new part of the fleet, while retaining the Cruiser class in probably a new ship.