This is where I am a bit confused. What do you mean? There are no physically weaker classes are there? I mean... is a Strength 20 Wizard... weaker than a Strength 20 Fighter? Like, I get that Fighters are more commonly Strength 20, but... if I have a Strength 20 Wizard and he can't wear full plate, I am really scratching my head. Interfering with spellcasting makes sense unless you MC one level of fighter...
I'm not sure any argument other than 'balance' and 'another D&D conceit' is going to convince me.