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.
Very few entities have a 98 year horizon. People sure don't. Some insurance companies do I suppose.
A more interesting graph would be to show me the 30 year return at each point along the way. My gues is that stocks would still mostly come out on top, but not the runaway you see here.
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.