Hindsight is 20/20 on the 60/40 portfolio.

That's what WisdomTree and Jeremy Siegel, professor of finance at the University of Pennsylvania's Wharton School, are emphasizing as they encourage investors to consider alternatives to the traditional 60% stock, 40% bond allocation.

The latest developments in their mission are two model portfolios made up entirely of exchange-traded funds: the Siegel-WisdomTree Global Equity Model Portfolio and the Siegel-WisdomTree Longevity Model Portfolio.

The two funds, offered first on TD Ameritrade's Model Market Center platform, take inspiration from two of Siegel's books — "Stocks for the Long Run" and "The Future for Investors" — in an effort to provide investors with higher returns in a declining interest-rate environment.

"We believe that the old 60/40 model just won't be able to cut it anymore," Siegel, who is also a senior investment strategy advisor at WisdomTree, said Monday on CNBC's "ETF Edge."

"This environment of low interest rates is not going to change," Siegel said, noting that the dividend yield on the S&P 500 is higher than the U.S. 10-year Treasury's 1.5% yield. "How is [that] ... going to give you enough income?"

The Siegel-WisdomTree funds will try to solve for that problem by offering low-cost, high-yield investment strategies that highlight one of Siegel's main arguments from "Stocks for the Long Run": that stocks may be subject to volatility in the short term, but ultimately have less long-term volatility when compared with bonds.

"That's why we recommend 75/25 as the equity/fixed-income allocation," he said, adding that it "would be the best way for those approaching retirement to establish their assets to get enough income and gains so they can maintain spending through retirement."

Global central banks have been partly to blame for the yearslong decline in interest rates as their monetary policies become more accommodative, but they're hardly the only driver of the phenomenon, the professor said.