Your point on battleships is true, but at the same time a battleship in a world where most people don't have aircraft at all would be incredibly powerful if it could be maintained properly. And really the thing people rather erroneously assume is that modern missile destroyers are in any  way analogous to WWII destroyers in scale. But again that would imply that modern warship classes actually mean anything (protip, they really don't). Who knows, it was the 2050s, these battleships could be things like the Russian Kirov in that they're huge missile boats, or they could be boats with large numbers of railguns made for super heavy support artillery. They could have point defense systems that means that only hyper velocity projectiles could damage them. I guess the point I'm making is that it is in no way hard to justify a battleship. 

I do concur about scaling down on the sizes of militaries as I had already said in an earlier post.