Back testing using historical equity returns has a major flaw - it assumes earnings growth in the future will resemble what we have seen in the past. Equity returns come come from dividends, earnings growth and change in multiples (PE ratio). It is more likely than not that we will not see same long term earnings growth in US that we experienced in the past. There is overwhelming empirical evidence that high levels of government debt lead to slow or negative economics growth. If there is no growth, equity investor will only get dividends and change in multiples. Chances that multiples will improve in the long run from the current high level are low.

If one assumes low growth environment, high yield bonds are very attractive. They will provide solid risk-adjusted returns in no growth environment, as well as in high growth environment if one's assumption about growth is incorrect.

Last point - to see how no-growth landscape looks like, look at Nikkei in 90s and 2000s. Depending on time frame of analysis, you will find low, flat or even negative returns over long periods of time with high volatility.