Besides, nobody takes AST over WHM for mitigation of all things.
AST is meta if people go for strong comps because it has higher rDPS and synergizes better with the even heavier focus on 2min burst since EW. If you want more mitigation than SCH or SGE alone can bring, you have your 4 dps and 2 tanks for it.

Barrier healers provide the same and occasionally even more raw healing via oGCDs than pure healers while also having mitigation. Which is why SGE/ SCH was so busted right from the start and still is while WHM/ AST struggled until late into the tier when everyone was overgeared and tank/ dps mitigation was enough to survive reliably.
The only reason SGE/ SCH isn't more common is because papa Yoshi said that 2 shield healers bad.
It's surprisingly easy to coordinate since only GCD shields conflict and SGE has Prognosis for this if GCD heal spam is really necessary and ET Succor alone doesn't cover it and their combined healer dps is sometimes even ahead of AST/ SCH.