That's exactly what I'm saying, in the late game, mages become unbeatable...

Unfortunately, you call this an imbalance, but in reality, it's part of the balance. The balance isn't about all factions having equal chances of winning at any stage of the game and under any conditions, but rather that under any conditions, all factions have the same chances of winning at the start of the game, and the result will depend on the correct choice of strategy. And, for your information, strategy isn't "we agreed with a friend to level up to 100500," but rather the correct use of all means, resources, and opportunities for victory in specific conditions. If rushing is advantageous, then you need to rush; if developing is advantageous, then develop. You have a one-sided understanding of the game.
And no one is arguing that after 10 months of development on large maps with one castle per player, the Academy faction is strong.