i don't agree with that point of view.
the white mage does miss in mitigation skills(shell and protect were always a white mage first core spells in ff),but shield isn't mitigation its more extra hp.
healers shouldn't be able to do what the other does,otherwise whats the point in different classes.
each class need to be with their own unique kit and be able to do the things they can do.
AST kit allow him to pick which he wants regen/heal before battle(probably a symbol of controlling and guiding fate) but the shield isn't omnipotent skill just absorbs a certain amount damage and i think its even smaller then what scholar grants also if i remember correctly SCH have their share of healing problems(not including that gigantic chaotic mess of a kit they have).
while AST and SCH have shield,it doesn't mean WHM need to be identical to them. WHM kit is more heal oriented,SCH is more shield oriented,AST is more of versatility/replacement.
WHM is(and always will be) the more straightforward healer, AST and SCH play style offer versatility and have more to offer then just shields/regens..
it definitely not mean that WHM suddenly need to be able to spam shield like those two,each class needs to stay true to what defines her and have their own unique take on healing and supporting the party.