I'm not sure what you're saying here. Nobody lost anything that stayed in the market. It fully recovered and then some. If you had continued to invest in the down market, you'd be doing fantastically today.
We're talking about different things I assume. My IRA(actually 2) is from old 401K's and I'm not currently contributing anything to either. So it goes up or down based on the performance of the investments. So when I decide the market is probably peaked and go cash, I'm trying to time the market. I fully realize I won't, it's impossible. But I'm also guessing that I will probably be able to decide when to get back in well enough to offset missing the turn.
With my Simple IRA at work, I'm still contributing so I don't move in and out.
For ages stay put was in fact the best advice but IMO that's no longer true. Since 99 we have had essentially 3 crashes, 99, the post 9/11 one, and then the Sub prime one.
During the sub prime one I went cash, many people I know didn't, most of them are ahead of what they were now but we're talking 250K, slide back to 170, now have 275K. They're ahead but would be much further ahead if they had gone cash at some point during the crash and then got back in later.
That's precisely what people who run funds do they get in and out of the market. as long as you realize you will never time the top or the bottom but you pay attention it's actually been fairly doable the last several years.
In a fairly stable market that is extremely hard to do, in a volatile market like we've been in the last decade or so, it's a lot easier to do.