Quick answer: extending a $1M retirement plan from age 90 to age 100 costs almost nothing at conservative withdrawal rates — success drops 0.1 points at 3% and 1.5 points at 4% — but at 5% success falls from 96.9% to 90.8%, and at 6% it collapses from 89.4% to 74.7%. The cliff sits right around a 5% withdrawal rate. Based on 105,000 simulations (21 scenarios × 5,000 runs, 60/35/5 portfolio, retire at 65).