Very true. Limited fuel supply contributed to the reduction in training time and the consequent deterioration in quality of German pilots and aircrew. This would be aggravated if they tried to field and maintain a large force of heavy bombers. And then there's that troublesome rule that you never get something for nothing. Resources invested in heavy bombers mean less of something else. It is true that before the war or earlier in the war they could have expanded their military production faster than they did; but they would still face the choice of devoting the new capacity to bombers or things like tactical air or motor vehicles that might serve them better. As I've said before, overrunning entire countries with mechanized forces supported by airpower is a form of strategic warfare. As others have noted, the Allied industries they could not reach with their air-ground team were mostly beyond the reach of 1940s heavy bombers also.