I'm reading The Intelligent Asset Allocator right now. Great book btw. Anyways, it was written in 1998 during the tech bubble. My version has no mention of the 2003 crash, the housing bubble, the 2007 crash, etc. But it talks a lot about the crashes and bear markets of 1990, 1987, 1973-74, 1929, etc. The author really points out to the whole problem of recency - ie we belive that what has happened in the last ten years is how it will always be. After every big bear market "things will never be the same", and if you are only comparing to the average of the last 20 year, you're right. But if you look at the mean over the past 70 years, yea, things tend to go back to the way they were (or close enough).
In the mid 80s bonds paid more than stocks (thanks to very high interest rates) and stocks would never return good again, watch out for run away inflation, it'll never be the same again!
If someone tells you they know the future, they're full of BS. If someone tells you "things will never be the same again", they simply aren't looking far enough ahead. your job, as your own money manager is to plan for decades, not quarters, and just as A Rebel Spy said, be flexible as you ride the rollercoaster of investing.