Infinite Mage is way more consistent than any shenanigans Big Priest will ever pull. I thought about that very same nerf and I could see that happening, or another one I thought of is change Bird battle cry to cast a random 6+ mana spell instead. It keeps any synergy with Mask of C'Thun and other big spells but no more infinite loop.
Btw, this is off topic sorta, but I hit legend in Wild with that infinite mage deck last night, and I was holding my infant son while playing and was making lots of mistakes (queuing the wrong cards for Commander Sivara, etc.), and the last two games the opponents conceded after I played Time Warp and then the first bird, even as I'm looking at my hand thinking "I won't be able to pull this off".
The deck is such a wretched thing to play against that people are throwing games they likely could have won if they stuck it out!