I do understand that you were referring to Tyranny, but casting limits have been a staple in almost all games. I don't want to get mired in the various secondary affects so lets switch the example to Minoletta Minor Missile. Straight up single target damage with no secondary. Using your example, with a Wizard and Ranger standing shoulder to shoulder in the back rank plinking away at the tanked mook, why is the mage done after a few casts but the Ranger is not? Maybe a system that offers spells that are unlimited and with spells that have limits is too weird?
I think at one time lower level spells were to become unlimited while the higher level ones would retain casting limits, but that plan went into the tank afaik.