Hey buddy you talking about champions having spells. all champions have spells and most champions use mana and all of them have less access to mana and mana sustain than mages.
Exactly, yes. But each class can do their job without huge amounts of mana, while mages can't do almost anything without it.
But you are forgetting that every champion type shares stats that the others use.
The thing is: For ADC abilities are supplementing his damage or maybe survivability.
But mages can't supplement their abilities with basic attacks as they are innately useless in AAing.
All classes share stats, but some are heavily reliant on one-two specific stats and so far only Mages are punished for being reliant on mana.
With your changes mages would use ammunition too or they would auto attack spending mana.
I'm not suggesting this change, god no. I'm trying to say that each class could have, as you said, "gating stereotypes", but in the end it's only Mages who have it.
Sigh.. You simply avoid understanding it.
For mage mana is like amunition for adc or stamina for fighter, but it's ONLY mage who has to live with mana-gating, as other classes do not have any gating on what they do.