Where are those oversized gains going to come from now?
Well.. stocks are not terribly expensive now, nor real cheap either. Perhaps we're due for some more 'normal' returns after the mess. A few years of steady unimpressive growth would be just fine with me.
In the next 10 years, the SP 500 could go up 10% annualized but your diversified portfolio could underperform T-Bills. As an example, real estate, commodity, and bonds loses could be negative drags wiping out your stock gains and giving you a 0% return annualized over the next 10 years. I suppose you do not think that is possible? A diversified portfolio may be logically very well thought out, but it is not a 100% sure money maker. Every method of investing has risk.
What you are saying is certainly possible, but unlikely. A review of the Callan table of investment returns and correlations data show that scenario is very unlikely. Correlations between different types of equities vary, but generally aren't negative... I guess what I'm saying is if US stocks do well, I would expect REITS to do well also, but not always move in tandem. Bonds generally do not lose money when stocks are up. Commodities are negatively correlated to stocks, so a strong bull market may result in weak commodity returns.
For all the asset classes you mentioned, they all have a positive expected return with varying correlations to the other ones.. so I'd expect a well-contructed portfolio to continue to be the best bet going forward - bubbles or no bubbles.
One last thing.. stocks MUST underperform bonds from time to time for there to support the existence of the equity risk premium over bonds. If stocks always returned more than bonds without downside risk, they would be safer and nobody would buy bonds. So consider times like this a necessary evil.
Bear Markets A Necessary Evil - Research