You're right, no one can choose the peak or the valley. After the bubble last time I can't count the number of people that said "oh, I knew there was a bubble". The problem was no one knew when it was going to burst. If you sold out in 2005, then you lost out on 2 or 3 years of growth because you mistimed it. Similarly, I had a couple people I knew that actually did time the stock market and sold pretty much at the peak. The problem is they were scared of getting back in and didn't buy in at the bottom. I actually did an analysis in like 2010 or 2011 that showed my continuous investing through the peaks and valleys was better than their cash in at a high and hold it all in cash.
I do think equities are getting over-valued. Take a look at
Shiller's CAPE ratio. The last time it got this high was April 2007, and we all know how that turned out...
My real worry is I can't figure out what's driving growth. It seems like the world was pinning its hopes on China growing, and it looks like they are starting to cool. In the US, I don't see any real sustainable improvement in the economy since the last bubble outside of fracking and smartphone apps. Maybe I'm pessimistic, but to quote my favorite tv show it feels like "winter is coming"