One bit you are missing is that it's entirely possible that today is the cheapest the S&P 500 will ever be. What if your 50% crash only comes after the market has tripled?
Another is this: What if the market crashes 40% but then goes back up? Do you buy at 40% down? What about 30% then?
It's easy to say "well I'll just buy at the bottom" but you can't know when the bottom is until well after it's happened.
The other major piece of the puzzle is that during a 30-50% crash, everyone you know and all the media will be screaming in your ears about how everyone is losing all their money in stocks, and that the only reasonable thing to do is sell now so you don't lose it all. Do you have the stomach to put all your money into the market in those conditions?
Another is this: What if the market crashes 40% but then goes back up? Do you buy at 40% down? What about 30% then?
It's easy to say "well I'll just buy at the bottom" but you can't know when the bottom is until well after it's happened.
The other major piece of the puzzle is that during a 30-50% crash, everyone you know and all the media will be screaming in your ears about how everyone is losing all their money in stocks, and that the only reasonable thing to do is sell now so you don't lose it all. Do you have the stomach to put all your money into the market in those conditions?