Not all mortality risk operates through cholesterol-dependent mechanisms. And even for those that do, decades of exposure to high cholesterol can’t be totally reversed by medications.
Ultimately you want a readout of CAD risk, and we know that only a portion of CAD risk is attributable to LDL. We're always on the lookout for better biomarkers, but right now there isn't a comprehensive CAD biomarker (aside, perhaps, from coronary calcium - but we can't go around exposing everyone to radiation every year just to observe disease progression).