The "one plane that can do it all" is the right choice from a strategic point of view. And all major wars are won or lost on the strategic level. Imagine two evenly matched countries going to war. Both countries have air forces of 100 aircraft, but their strategies are different: Country A has developed one plane that can do it all, albeit not as good in any one role as a specialized singe role aircraft. Country B has developed specialized aircraft for each role. They have 25 air superiority fighters, 25 bombers, 25 close air support planes, and 25 recce/ESM/ECM planes.
Day one: Country B's 25 air superiority fighters face off against 100 multi-role fighter/bombers in A2A config and lose... Badly. Though their specialized fighters are somewhat better pound for pound, they can't handle 4-1 odds. The rest of country B's air force is destroyed in short order having lost its fighter component.
Day two: Country A's surviving multi-role aircraft switch roles to recce and air-to-mud and operate unopposed against enemy ground forces and strategic/political targets.
Country B loses the war.