I don't think the solution to bring WHM/SCH back to viability is to nerf ASTs or AST's Balance. You shouldn't nerf a job to make the others better. Nerf Balance and AST will go back to being the weaker hybrid groups didn't want because they couldn't bring anything to the group the other two didn't already bring.

Each job needs their own niche to make them viable for any content, to make them unique, and to make them desirable to statics and players in general. Nerfing one will only cause said nerfed job to experience problems; it won't necessarily fix the design flaws with others.

There are definitely aspects of WHM that need to be addressed. And SCH needs A LOT of TLC right now. But nerfing AST is not the solution to fixing WHM/SCH. Just like how nerfing PLD wouldn't be a solution to fix the issues with WAR, or even DRK.