1. Not all broad indexes are great buy because lot of companies will do great in this crises but others will suffer long term. Instead of investing only into S&P 500, you might want to also diversify in Europe, Asia Pacific, China markets as well as have ETFs in energy, utilities, finance etc.
2. S&P500 doesn't actually have all that great returns, contrary to usual belief for same risk taking. For example, QQQ has returned ~30% avg/yr while SPY has only ~12%/yr over past 10 years even at lowest points. This is more than double the difference for reasonably similar level of volatility and liquidity!
3. The buy and forget strategy misses out on entirely on "unreasonably low" prices event. For example, massive ETFs like XLE are at all time lowest price and very unreasonably low priced. Investing them in now can boost your returns massively.