I personally think AST will play more into buffing DPS rather than doing the DPS, obviously it will still be capable, but I don't nescessarily see it having SCH's niche. I think SCH/AST will be amazing offencively paired (Selene/Cleric and Cards and should shields stack, huge barriers making old content easier due to gear scaling giving both time to dps) and WHM/AST will be very defencive (Big heals, paired with buffs and Astro shielding).

We have to bare in mind that WHM has Proshell and Improved Stoneskin, SCH has Adlo crits, Fey Covenant, Fey Illumination, Glow and Light, Virus and Sacred Soil. (of course there is more to mention)

Astrologian won't replace the other two healers, but instead strengthen them, should they make card buffs overwrite each other and shields not stack or w/e, it would make stacking AST not entirely ideal, I think the healers will all benefit well... but imagine having all 3 in one party! xD