To me, the real tension is not whether or not Ash could defeat Alain and his Mega Charizard X, but rather whether or not Alain would realize that he's been played. The most dangerous aspects about Alain are his misguided belief about power and his obliviousness to Team Flare's true colors. He is passionate to his mission, and thus dangerous if he's on the wrong side. But make no mistake, Alain is not a classical villain. He's an antagonist but at his worst, he's just an anti-villain trying believing he is doing good instead of harm. Simply defeating Alain in the Team Flare conflict will not be enough. He can be defeated but never learn his lesson. In order for Alain's character arc to come full circle and for Ash to truly defeat him is to show him the truth of Team Flare. And if successful, Alain will switch sides. No need for a grudge battle.
I stated this before. If Alain were to lose, he might take the wrong lesson from the loss and become convinced he is not strong enough, thus falling deeper into Team Flare's dark influence and taking more villainous actions (like kidnapping Squishy). Then Ash has to show him what he had failed to learn. Alain winning the League does not really accomplish anything other than making him a more formidable challenge to Ash in the Team Flare conflict. But given what we know already, Ash doesn't really need to challenge Alain into another battle if Alain is simply misguided. He needs to simply show what Team Flare really is and that will be easy given that Lysandre won't need Alain anymore.