The dead person can't make bad decisions on when to buy/sell the index fund. Those "bad" decisions don't even require an attempt to time the market, if e.g. you

The dead person can't make bad decisions on when to buy/sell the index fund. Those "bad" decisions don't even require an attempt to time the market, if e.g. you always invest whatever's left over after ~fixed living expenses, and you get paid more when the market's higher.