The advocates of the 60/40 balanced portfolio forget that the 40% that is invested in bonds poses a great risk for no return. Given where bond yields (if you want to call the puny returns on offer a 'yield') are the only way that bond prices will ever move to a significant extent is DOWN. That would mean losses on the bond portion of the portfolio. Much better to stay in cash (or pull out the money from the bank if ever the governments impose negative interest rates).
Vanguard defends 60/40 Portfolio
Harvard Endowment Underperformance, part MMXXIV
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