Personally I remember people who didn't sign up for BowMage in Heavensward rioting and I see this Half-AST casting as an affront in a similar manner. It's not the playstyle I signed up for, and unlike HW I'm not getting anything significant back for it. SCH players often have good reason to double-weave their oGCD actions due to pet constraints, Aetherflow management, and general fight tempo, and I don't see any of that going away in Endwalker. There's going to be situations where I'm gonna pop Fairy cooldowns, and that slight delay you're outright ignoring will get someone killed a few seconds down the line due to not being able to early-weave the action. Are they rare once you figure out a fight? Sure, but it's also not enough of a reason to remove Ruin II for the people who actually use it to their advantage in prog.
For WHM it's undeniably a buff. For SCH it's pointless unless they remove Ruin II outright, where it's then an outright QoL nerf.