I bet someday we realize that "causality" is about as real as the ether or force at the subatomic level. That is, something that seems fundamental and obvious but is really just wrong.
The most popular framework for causality talks about potential outcomes. If I took my blood pressure meds now (all else equal), what would my blood pressure be at noon? If I skipped my meds today (all else equal), what would my blood pressure be at noon? That tells me the casual effect of the meds on my blood pressure today. But I either will or won't take the meds. One of those branches is simply wrong on a fundamental level, like you say. "What would happen" is in some sense meaningless. But it's sure as shit useful to think about, and that's the whole point.
Said differently, even in a toy model of the universe where everything is deterministic and causality is meaningless on a physics level, causality can be usefully defined on a practical level. Counterfactuals are counter factual, but that's ok.