Old Cleric Stance was bs, sure mechanics are predictable but you would still need to clear the dungeon/trial multiple times before you have it somewhat memorized, and can effectively start using Cleric Stance.

The bigger issue is that while mechanics are predictable, players in PUGs aren't, you do your usual groundwork and give the tank regen/mitigation you always do before switching to Cleric Stance to get some DPS in, only for the tank to not use their tools and die anyway. That, or the DPS players in your group getting hit by something as you switch to Cleric Stance, and then having no way to save them due to you being stuck in Cleric Stance for a bit.

And personally, I always saw Cleric Stance as pointless busywork, having to press a button to get into and out of "DPS mode" doesn't suddenly make playing a healer interesting.