3 ideas
Livy argues that Alexander was one man with one life, while Rome had many generals of comparable ability who rotated through command and could replace one another after a defeat. A state that produces a steady supply of competent leaders can absorb losses that a system dependent on one exceptional leader cannot survive. Rome lost battles but never a war, because its capacity for leadership regenerated.
Livy notes that Alexander died young and undefeated, so his reputation was never tested by the reversals that long-lived commanders suffer. He points to Cyrus and Pompey, whose fortunes turned later, and argues that luck and early death give short careers an illusion of invincibility. To judge any leader's greatness fairly, you must account for how long they were exposed to the chance of failure.
Livy does not simply assert that Rome would have won. He breaks the question into comparable factors: the quality of commanders, the number and discipline of soldiers, tactics such as the legion against the phalanx, and the ability to replace losses. He also argues that Alexander had grown softened by Persian luxury, and he tests each factor in turn. Breaking a what-if question into separate, weighable variables turns speculation into an argument that others can check.