[Aeraaa]

^In addition to that, there's the rather obvious detail that you cannot perform your duty/mission while you're dead (or shot down, which is the same for all intents or purposes). So, if you've encountered stronger resistance than expected, retreating is the most sane thing to do.

That isn't to say that retreating does not entail danger of its own. I mean yeah, you CAN potentially be held accountable for not completing your mission depending on the army you serve, the attitude of your superior officer etc. However, few nations are willing to expend pilots for a successful mission. After all, planes can be send to battle again if they are still flyable.