I just understand why in WW2...

that while Axies and Soviets prefer to use dedicated ground attack planes, the westen Allies mainly used fighter-bombers, which are mainly secondline or modified fighters, for air support mission.

That's because...
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Under Democracy, dedicated ground attack planes creat unhappy citizens, while those with fighter role don't.

Surely I know that the true reason in history was, the Allies have overall better engine and airframe technology and more resource to make their fighters capable to lift heavy payload and absorb more damage(late war P-47, Tempest), or simplly make more of them to take casaulties(early war Harrican), which in turn make them good fighter-bombers.
But I do think we can make so in the next WW2 scn, to present Allies' advantage. A typical Air superiorty Fighter/Fighter-Bomber comparasion would be that, they have same attack value (FBs have more guns but worse manuverbility so roughly compensated each other), the later ones have lower defence value and lower cost. Air superiorty Fighters can be used in ground support role (as it was always in WW2 history) but less cost effective.