This is really underappreciated. I'm a big fan of diversification, and it can be very important to lower your risk-appetite if you're relying on your investment income in a short-medium timeframe (e.g. if you're planning on retiring in 5 years, being all-in equity is just silly, or if you're saving for a downpayment on a home in a few years, stock investing isn't the primary instrument to use), but in the long-term (e.g. a 25 year old thinking about early retirement someday) it just doesn't make sense to take too little risk.
And in part it's precisely because of the worst case scenario happens where worldwide stocks don't outperform treasury bonds, you're likely living in a world where your money isn't legal tender anymore anyway, and any stock/bond investment decision you ever made basically irrelevant.
So if decisions regarding the extreme downside risk are irrelevant, you might as well optimize for the upside.