But the S&P500 has crashed, a few times in recent memory. It doesn't have to go to 0, it just needs to not be a perfect bull market all then time. If you're at 80/20 stocks/bonds and the market crashes your allocation might shift to say 70/30 so when you rebalance you're selling bonds to buy stocks. Its a way to perpetually buy low and sell high regardless of what the market does.
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But the S&P500 has crashed, a few times in recent memory. It doesn't have to go to 0, it just needs to not be a perfect bull market all then time. If you're at 80/20 stocks/bonds and the market crashes your allocation might shift to say 70/30 so when you rebalance you're selling bonds to buy stocks. Its a way to perpetually buy low and sell high regardless of what the market does.