What you are talking about the mythic levels being better for casting than the stat is wrong because of the pace you get these levels and the spell levels you get for them. Your spell level is considered 2 for the ability you get at mythic level 3. So you are getting spells that are cast as level 2 spells with 3 bonus to it. They are equivalent to a level 10 character casting a level 2 ability with +3 stats. You have this bonus for nearly entirety of act 3, which is the longest act, and has enemies from anywhere from 10 to 18.
Case you are describing is impossible - Azata gets lvl 2 spells at rank 4, she could never cast them with "3 bonus to it". She gets at lvl 4 and that is analogue to 18 main spellcasting stat. So replacing casting stat with Mythic Rank we get "casting" stat in +3-10 range, analogue to 16-30 Int/Wis/Cha. It is more than 95% of builds, who are not spellcasting-focused could ever dream of.
And I agree that Mythic Spells are useless on Hard+. Fuck, half of my Bard's spellbook was useless on Unfair in P:K. Should we delete it too?