Andor said:
Spells do run out, However flaming, shocking, thunderous weapons do not, and neither Magic shields and Armour. A magic-weilding societies fighters will chew through those mobs like a buzzsaw through wet paper. Not to mention summoned outsiders, undead, charmed lycanthropes, etc. Plus we have healing and ressurection, and you don't. It's like asking a stone age society to hold off a modern one.
Unless you're using nukes or similar weapons (few spells come close, and per D&D stats few people have those abilities) numbers count. Even in D&D, fighters can't take 100 to 1 odds. Add trip attacks (a touch attack), pits, hot oils, and other tactics and the armor doesn't count for much.
Making those weapons leaves your spells casters a lower casting capacity, so you have to decide; have all the fighters, or all the spells.
Unless your dealing with advanced undead, they won't handle a mob. Skeletons are too brittle, and zombies too slow. Even if you use those that create spawn, that's only effective in prolonged battle. It still isn't a guarantee.
Charming anything doesn't make them suicidal.
Healing is limited per day (provided your gods agree with the battle). There's a good change that you have fewer clerics than fighters, so attrition is inevitable. Ressurection requires supplies, of which will most likely not be enough to raise the whole army. Matter of fact, if the non-mages cut off supply lines, you lose spell components. If you rely on planar travel to gain them, you lose out on spells per day. If you stock piled those supplies ahead of time, hope the non-mages don't flaming balista the sulfer supply...
The amount of outsiders you can call are limited. Either you summon many small ones, which can be dealt with with high damage rolls (laws of probability dictate that more than a few will be rolled), or you summon larger ones (who can't call friends) whose abilities are limited to a single vector (much like your mages). Summoned creatures are also around for a limited time, so non mages can wait them out. Mages cannot say the same for the non-magic users.
Summon spells are extremely limited in duration. At best, you get a high level outsider for just over 3 minutes. Anything with a higher duration requires a greater cost. You'll severely weaken our casters over time (exp costs for spells), and I hope you like being beholden to outsiders for a good long time...
Spells themselves have limited durations and ranges. How many fireballs/meteor storms would it take to kill Souran's army during the last battle in LotR?