Every position has it's vulnerabilities though. I think it's mostly just because there's so many damn skills in MMA that a fighter needs to learn, so it's just more time efficient to learn how to wrestle up better. I don't think that means high level guards are pointless though or even necessarily inferior approach.. I know you didn't say they're pointless by the way, I'm just rambling.
At some point, having the common platitude of "guard being worst case scenario" becomes a self fulfilling prophecy, because people end up believing this and neglecting training guard work, and then when they find themselves on their back and can't wrestle up, they are indeed fucked after having treated guard work like some kind of extra credit in the gym.