Once average Americans discovered Wall Street, they embraced it. Based on average returns over the last 100 years, stocks have beaten every other asset. Younger people just want equities because they have learned it is no more risky than fixed income over time. Defined benefit pension plans are becoming more unusual, and Americans must assume more responsibility for their own retirement funds. As baby boomers approach retirement, they are accelerating their investments.
Seasoned investors are more comfortable with market fluctuations.
These factors will all continue. Even as baby boomers retire, they won't take all their money out of the market. If there is a downside, it's the youthful investor who sees exuberant market performance as the norm. It was a spotty market in 1999. That gave them some classroom experience riding the waves.