The thing is
1 you dont have to nuke the entire army to defeat it - just enough of them to have the others break (there needs to be morale rules for DnD armies)
2 you dont need an army of mages to break morale - just a couple using cloudkill or chain lightning or such should be enough, not to mention create undead etc etc
That's why I defaulted back to Warhammer and old D&D frequency rules.
In a full on battle, you don't have enough mages to rout an army. You can damage a few groups but a noble wouldn't have enough mages with enough power to be more than an artillery group.
Base D&D worlds don't have that many 7th+ level casters in a given nation for hire. And with volley fire, low level combat casters would be pincushions. That's why many fantasy miniature wargames and videogames have mages as character snipers and walking buffs. You fireball the enemy leadership.