Cause 500k frostbolt is fine, right?
500k Frostbolts are fine since it's only with Ice Form and Deep Shatter. Normal Frostbolts do 100k and Normal Deep Shatters do 300k. You can do 2 guaranteed Deep Shatters with Pet Nova & Ice Nova, some times you can blink in for a Frost Nova or you get a Flurry procc so you can get an extra Shatter.
Mages give up Fingers of Frost for those 2-3 500k Frostbolts per minute.
10% Stat nerfs are bad in general, not just for Mages. It's a lazy way of balancing the game where you risk over nerfing.