As pointed out by Shiller (“Irrational Exuberance”, 2000), equity risk premia won’t always be positive. However, that is only a straw man; nobody actually claims that.

During 1985-2010, equity returns were considerably lower than during 1952-1984, materially affecting the risk premia. As shown in the table below, over periods of 15 years, based upon simple UK return differences between equities and long conventional gilts, the risk premium would be negative 16% of the time during the early period and 25% of the time during the later period. 

Taking the blend column (80% chance of being in the later period), the risk premium (% pa) would be higher than 1% 69.5% of the time and higher than 2% 62.0% of the time, with 3% being slightly more challenging (better than evens). So long as one has enough time, these seem pretty good odds.