Against Manaless mids, you have to use your spells or be pushed under tower all laning phase. "just learn to last hit under tower bro" And then be susceptible to dives, lose all lane pressure, and basically give them free reign to roam.
Why in the world would I buy a second Doran's Ring if the passive doesn't stack. 60 HP, 50% Mana Regen is not worth it. Might as well buy Dark Seal instead for the potion healing and flat Mana, plus it has a higher resale cost.
Only 4 (not 5) Mana per mininion? That's 24 Mana per wave. 28 on a Cannon wave. That's ~50 Mana per minute with perfect CS. Mid laners never created issues with stacking Rings, just Tanks.
And yes, it will change the meta slightly. Early build paths will change.
"Just counterpick bro" Not everyone has the luxury of picking after them, knowing the champion to counter them, or even own a champion to counter them.