If you've got a continent with five realms, three allies on one side and two on the other, and for various reasons four of these realms are led by peace-loving rulers that maintain the status quo against restless nobles that would like for something to happen. Now say that one other realm, with a single ally, is the only one ruled by a ruler that wants to shake things up. So that ruler tries to prepare his realm for war, he orders recruitment of troops and investments in recruitment centres. But, despite how wealthy his realm should be, none of that seems to be happening. His army remains small and the infrastructure remains poor, because those with gold have other priorities. He can't ban them, they'll just switch to one of the enemies, and he'll just be less able to go to war. And so, because he can't mount a reasonable army, he puts off the war again, and again, and again. Just because one or two douchebags wanted to use that gold to train their skills, the whole continent is at peace. Had he been able to divert all of these resources to a war effort, he could have launched a surprise attack on one of his enemies, their two allies would probably have come in to defend him, and then his own ally would probably have come in to even the odds. That single ruler could have brought the whole continent at war.
Academies are detrimental to the game as a whole, not just to the realms that use them.
I'm sorry, but your argument boils down to "I know how that game should be played and I want everyone else to play the same way". If you think an academy can weaken a realm so much, then you can be the one to take the lead and attack those realms with academies. Your own academy-free realm won't have these problems, so it should be beat them easily.
Hey, I can even turn the argument around. If academies were not available in any realm, then there would be nowhere to spend your gold but in the military. This would result in all realms being armed to the teeth and would lead to a long-term peace due to the equality of forces and mutually-assured destruction.
