Why has the concept of battleships/pocket battleships been wiped out of modern military warfare?
The vulnerability of unescorted battleships to air attack is not the reason. A battleship could be part of a carrier group, protected by the carrier's air umbrella, and be at no more risk than the carrier itself, besides being a smaller target. (Test run of this argument: the advent of torpedoes did not make battleships obsolete, but did mean that they needed a screen of destroyers.)
The true reason is that big guns are no longer the longest-range weapons afloat, nor do they deliver the heaviest warheads. Since the 1940s, naval battles are fought at ranges of hundreds of km using aircraft and guided missiles, or if at close range, stealthily using torpedoes and depth charges. I hope I don't need to elaborate on why it's hard to fire a 16-inch shell stealthily from underwater. More importantly, the long ranges of aircraft and missiles have made 16-inch shells as irrelevant as pikes and cutlasses became during the 19th century. If an opposing ship is close enough to you that you can use these weapons, the fight is already over. As for thick armour, conventional bombs can be built far bigger and deadlier than a 16-inch shell, so a ship that could "just sit there and take it" would have to be about as big as a shopping mall or football stadium, and would not be much more manoeuvrable. The best defence (for a warship) really is attack, or sometimes escape.
