This idea that when the next recession hits the market will lose half its value is insanity. Look at how much value the market lost in most of the major crashes in history. Greater than 50% is a once-in-a-lifetime event. Even considering the 2008 crash, if you sold the S&P500 in early 2007, a bit before the crash (value ~1400), and bought in late 2009, a bit after the bottom (value ~1050), you only got a 33% return. Nothing to sneeze at, but that's taking into consideration that you actually just timed the worst market crash in modern history.
What's more likely to happen is that you sell (or fail to buy) today at ~2400, the market wiggles around for a while, goes to 2600 or 2800 or 3000, then has a minor correction down to 2500 or 2700 or whatever, and then continues a slow and steady climb. You just don't know which number will be the top and which will be the bottom, but you do know that the slow and steady climb will resume and continue for decades to come. In order to 'time the market', you have to guess BOTH the top AND the bottom. And it's very likely that the 'bottom' (assuming you can even time the bottom correctly) will end up being higher than the price you can get today, right now.
Sorry for actual words....