The point the OP is making is that given similar circumstances like equivalent amount of phy/magical armor, magic will by default be at a disadvantage DPS wise thanks to elemental resistances. Given this, why bother bringing magic characters then since the disparity between physical and magical armor characters merely offset magic's in built weakness in the best of cases but highlights magic's weakness in the majority of cases.
Just consider these scenarios,
If you meet a hypothetical mob like maybe a Fire Mage with 100 magic armor/ 50 physical armor and maybe 50% fire resist with 100 hp.
A physical dps who does 25 damage per 2 ap would finish the fight in 12 ap.
A magical dps who does 25 fire damage per 2 ap would finish the fight in 16 ap without factoring the resist. I am not sure how it affects armor/hp but lets assume its a flat reduction for illustration purposes, that means you need 24 ap.
If its a Fire Warrior with 50 magic/ 100 physical armor and 100 hp with 50% fire resist.
The same physical dps would only need 16 ap compared to the previous case of 24ap.
The same magical dps would need 12+6 ap which already makes it worse than the physical dps in the same scenario.
While the above case is biased because the magical dps is using fire to deal with a fire resistant mob, let's be honest here. Most of the other trees do not deal that much damage either by default or require Fire as an intermediary. Therefore, using Fire to deal with a fire resistant mob would more than likely be the common case. In contrast physical dps would simply plow through everything without any issue while still having the same utility as magic dps in terms of cc.
While there could be cases of mobs weak to fire, the lack of an equivalent alternative when faced with a fire resistant mobs means that these 2 cases do not cancel each other out and instead makes magic inherently weaker. Plus there are cases where mobs are just flat out immune to fire which is a scenario physical dps do not face at all.
Mobs require physical resistances as stated by the OP but to a lesser extent in order to balance this out otherwise there is really no reason to bring a mage especially when warfare is able to aoe pretty well while ccing.