P-tau217 emerges as the strongest predictor for whether cognitive decline will occur, achieving AUCs up to 0.81 for forecasting dementia risk within 2-10 years in a cohort of 4,073 adults followed for nearly three decades. Individuals with above-median p-tau217 levels faced 2.57-fold higher odds of decline within two years, escalating to over 10-fold increased risk at the decade mark. Meanwhile, p-tau181 and GFAP proved superior for tracking the rate and severity of ongoing decline across all time intervals. This represents a critical advance in dementia risk stratification, as plasma biomarkers have historically excelled at detecting existing Alzheimer's pathology but struggled to predict future cognitive trajectories. The findings suggest a two-tier screening approach: p-tau217 for identifying who will decline, and p-tau181/GFAP for monitoring progression velocity once decline begins. The 29-year follow-up period provides unprecedented longitudinal validation, addressing previous concerns about short-term predictive studies. For clinical practice, this could enable earlier intervention windows and more precise patient counseling. However, the cohort's demographic homogeneity (90% non-Hispanic White) limits generalizability, and the observational design cannot establish whether biomarker-guided interventions would alter outcomes.