An even darker thought: even "the good case" is built on the idea that you can effectively hit 8%+ risk free returns with a reasonably diversified portfolio.
This is absolutely a valid concern. The stock markets have been effectively stagnant for the past 10 years. Sure, we had a good run between 1930-2000, but long-term patterns are as unpredictable as short-term ones.
What if that is no longer the case?