What the Germans can concentrate, and where exactly? They historically fielded much smaller number of fighters than either of the UK, USA or USSR, the heavy Flak is already concentrated around production centers, and thousands of those were historically unable to thwart any WAllied bombing offensive on their own.
Let's also not asume that Allied leaders are stupid. The very moment they realize that other German assets are defeseless, they will plaster those assets. Say, 14 days of POL offensive, 2 days attacking electrical production & distribution centres, 2 days chemical factories, then again 7 days against POL, 2 days vs. marshaling yards. They can take advantage of Flak forces being unable to shift into defense as fast as the fighters can.