People are making the mistake of comparing an 18% stoneskin to a scholar's adlo. The choice is never between a white mage or scholar. The comparison which should be drawn is that between a whms 18% and an astros 10% stoneskin. As it stands, they both have the same mitigation. But changing the whms to 18% significantly tips the whm to be superior by a huge margin. It adds a big reason for people to take whm over astro (see: uplander doom) which is exactly what se said they didn't want to happen. It would give whms a bit too much utility on top of what they already have.

And on a side, people are incredibly underestimating schs aoe potential. Succor + indom is 700 potency worth of heals in 3 secs and the option of emergency tactics is there if necessary, as is deployment tactics and whispering dawn.