I don't really see it as an overreaction. They are more concerns.
SE already said it themselves. That even after the buff, they are going to continue to monitor Astro. For all you know, they can be nerfed or buffed again. Buffed being highly unlikely.

I know that they are trying to make it so Astro can be a viable healer, but they are doing it by simply giving them the best of everything from the other 2 healing classes.

WHM has always been known for it's group healing ability and high potency heals.
SCH is known for it's mitigation.

Now they give Astro the best mitigation ontop of giving them the healing potencies of a WHM WHILE also maintaining Astro lowered natural MP cost of healing spells.

The only unique healing class is Astro now. If Astro can main heal and buff group dps, why would you bring a WHM? They can only heal. They can't debuff enemies or buff allies.