They're still pretty durable. Germans, for example, build 8 huge concrete towers for AA defense, and those things were so damn durable that Soviets couldn't destroy them even with sustained artillery fire, nevermind the RAF bombers. Those things also served as air raid shelters, and indeed, were the safest places during the siege of Berlin (they were also the last places to surrender).

So, they were only mostly useless, and there's a big difference between mostly useless and all useless.