I'm glad I don't have to worry about such things any more.
See my discussion of the collar strategy in Investor Alley. A major run-up would be profitable for me, and a major collapse would be a buying opportunity. I've set up positions so that both statements can be true simultaneously, and a ratcheting effect locks in my gains as the market moves up over the years. I'm still exposed to the risk of a dollar decline, though.
That said, I'm not going to avoid the question like everyone else is doing.
There have to be circumstances for any individual in which it is rational to exit risky assets. Otherwise we are advocating blind faith. If you say "I would buy US stocks no matter what" isn't that like having an unfalsifiable opinion?
Even the most staunch advocate of B&H-no-matter-what is not piling all-in on Chinese stocks, or Argentine bonds, or Turkish stocks, or Russian ruble currency arbitrage, or the latest buttcoin, or orange juice futures. The no-matter-what crowd would say these markets are too risky - far riskier than B&H'ing the S&P500 has historically been. But what if something occurred that made the S&P500 more risky than it has historically been?
We can debate the particular circumstances, but first we have to establish that such circumstances exist? Does anyone here think it impossible for the U.S. stock market to become too risky?