RIP Astrologian

[Sylve]

Because Astro may not be able to provide big heals or heavy shielding with their barrier, it's like the RDM of the healers basically... It can do both but it's not great at either. .-.

Keep in mind that RDM never ever replaced BLM or WHM nor sword users just because they were able to do all three. Rather they were a side asset to them, much like astro would probably be with their cards.

If the AST had lower potency heals than SCHs and WHMs, they'd struggle in Light Parties where they were the sole healer.
SCH works out fine because Adloq = Cure II when accounting for the Shield and Eos is free spam healing.

The RDM didn't replace the BLM and WHM nor sword users because those Jobs brought something to teh table that the RDM didn't, hence why it supplemented them rather than outright replaced them.
Do you really think the BLMs would get a place in XI parties if RDMs had the exact same offensive power in their spells while also providing healing and buffs? I think not.

That assumes a lot. A large part of why people avoid SCH+SCH right now is because Galvanize doesn't stack at all, so what makes you think AST shields will stack? Especially given that SE's response to SCH/SCH not stacking is to let one of them go pure heal? And we've already been told they don't have the throughput of the other two classes, so taking two healers who don't have throughput would be a bad idea.

And yet WHM+WHM is still allowed to stack.

The issue wouldn't be with the stacking of shields, its the ability to provide both shielding AND big heals at the same time.
It'd be like taking 2 WHMs and getting a free 9th slot SCH in your raid team.
You would have the ability to provide 2x large heals in addition to being able to suppress damage via shields when required.

If both ASTs stayed in WHM mode with one of them designated to switch to Barrier before any major AoEs to prep shields and then goes back to WHM Mode to provide those bursty heals, you essentially get 3 healers worth of healing in.