Exactly, if it had been obvious at the time that "the market" would deliver a better return, for certain, then nobody would have bought bonds at those prices.
Then bond prices would have declined (and their expected returns or interest rate would have increased) until, in equilibrium, the anticipation was that the stocks and bonds would deliver comparable expected risk-adjusted returns.
Then bond prices would have declined (and their expected returns or interest rate would have increased) until, in equilibrium, the anticipation was that the stocks and bonds would deliver comparable expected risk-adjusted returns.