GrinBird,
The only way to enforce your suggested rule would be to make bombsites inaccurate. If this were done, we'd never see buffs in the MA anymore. We don't have enough strategic stuff to hit, nor does what we have incur enough of a penalty on the enemy to justify much use of heavy bombers.
If we had factories that produced the various aircraft; say 1 factory per side for outdated stuff like Spitfire MkVb, C.202, A6M5b, Bf109F-4 and La5s, 2 factories for mid-range stuff like Ju88, Lancaster, Typhoon, B-26 and C.205s, 3 for high end stuff; then the heavy bombers could actually have a noticable effect on the war without pinpoint destruction of airfields. Under my idea, the last available unit of a given type could always be produced, for example a side could never be reduced to no bombers, no fighters, no tanks and no transports. If the last fighter available to one side is a Bf109G-2, and the factory that builds it is then destroyed or captured, that side could still build Bf109G-2s.
Sisu
-Karnak