Wizardry is genetic, the humans could just invent a drug or virus to turn wizards into humans, like the 'cure' in X-Men, or make all the wizards infertile.
Wizards are only good at wizard stuff, and people are kidding themselves if they think an army couldn't beat a shit ton of wizards who aren't even trained in combat. All you learn as a wizard is basic self defense against other weak magic spells, how to levitate feathers and fix broken glass. The average wizard could probably be killed by an unarmed fighter lol.