Not sure I follow.
The existence of an equity risk premium is an observable historic fact. Investors demand compensation for taking risk - otherwise they would just invest in risk-free assets.
The effect of "time diversification" doesn't make stocks less risky, it just increases the probability that the risk will be compensated. Put another way, it "averages out" the occasional negative outcomes of shorter holding periods.
The existence of an equity risk premium is an observable historic fact. Investors demand compensation for taking risk - otherwise they would just invest in risk-free assets.
The effect of "time diversification" doesn't make stocks less risky, it just increases the probability that the risk will be compensated. Put another way, it "averages out" the occasional negative outcomes of shorter holding periods.