The trade-off for that though is the RNG, I think. I will have some runs where I get Balance after Balance, and other runs where RNG says "Oh, I heard you really wanted some Spears." Really, they could rework Spears to make them useful for more than just Royal Road/Minor Arcana burning. Instead of limited CD reduction to CDs used during the duration, maybe adjust to 10% reduction on CD timers on party members outright (5% if AOE). Not too OP (20% single/10% AOE outright could possibly border on OP), but now not nearly as useless. Only time I use Spear is on myself for Lucid Dreaming timers. If I have one at the ready when I need to pop Lucid. If they changed Spear to where it just flat-out reduced CD timers of skills already on CD, it would be useful to throw on tanks to allow them to get their mitigation CDs back faster. Especially on tanks like DRK where certain mitigation skills are very situational (Dark Mind), and they only have a couple solid ones for all content (Rampart and Shadow Wall, for example).
They could look at other cards to try and make them more viable and more desirable, but everyone is going to desire DPS increases in a game where the playerbase cares so much about numbers, so Balance will always be the card people care the most about. There isn't really a way around that other than nerfing it to the point where no one cares. And if they don't add viability to the other cards, AST will go back to 3.0 AST--undesirable because they don't bring anything the other two don't already have. That doesn't solve anything. Just makes another job gimped where as the others aren't. One shouldn't have to suffer at the expense of "fixing" the others.
I like the fact that I can buff on AST, and I don't really want them to change the buffs on AST. Rather they need to look at the other two healers and buff them; give them things that AST cannot bring to the table, be it more powerful HoT/HPS, or more potent/longer-lasting shields, or better mitigation. Fix SCH's fairy to where Chocobo Companions DON'T out-heal them. Give SCH fairy maybe different buffs (something other than Fey Wind/Fey Illumination to make them desirable). Expand on WHM Lilies maybe (can't speak much on WHM right now since I haven't looked into the whole Lily/Confession thing yet). Something to give healers their own niche rather than taking away from one healer because they can't figure out what to do with the other two to bring them up to speed.
For those saying AST needs reworked: how are you wanting SE to rework the job? The idea behind ASTs was the card buffs, little weaker HoTs/HPS, little weaker shields (though right now Noct shields are better than SCH even with Noct losing it's heal potency increase--SCH definitely needs to be looked at and buffed to bring it back to viability). What rework would you propose to make all three healer jobs viable in some way while also maintaining a sense of uniqueness about them all?
Before when ASTs were garbage, they had nothing to bring that the other two didn't already have. Their buffs' effects were negligible, and the Draw design was clunky at 3.0 launch until SE fixed it. Then SE buffed the cards (Balance in particular), and now AST is suddenly too OP and needs to be nerfed? Nerfing AST buffs will just make it back into a weaker version of WHM/SCH with nothing to bring to a group that matters.
The thing that needs to be done is not nerf one job to make the others viable. Rather the other jobs could be buffed instead to give them their own niche back. AST's niche is its cards, and it took SE basically an entire expansion to get AST into a good place (and really, I sometimes think they still aren't entirely sure what to do with the job sometimes). SE needs to give the other two healers tools that AST doesn't have; tools to give them a uniqueness and make them appealing to a group/static setting. The solution should never be to nerf one job so that the others can be viable again.