All of this revolves around the belief that Theodoric ends up with a stable line of succession with heirs that don’t die. Also, Justinian would either have to not come to power or be so distracted by the Sassanids that he would be too focused on the East to match West at some point.
In all fairness, the two do complement each other: it was the dynastic instability that gave Justinian his pretext to invade. If the Analings keep a series of stable, pro-Constaninople rulers on the throne for a couple generations, they’re very well-positioned. Hell, given that Athalric is young, you have the potential for a very long reign.