If one is equally likely to miss both good and bad days, but overall the market is up 10%+ in a year (repeated over decades), then money left in the market will double every 7 years or so (Rule of 72 [1]). In that way it is almost always preferable to invest and hold in index funds.

The S&P 500 has returned about 13.6% return per year from 2010-2020 [2], or doubling your money about every 5.5 years. If that rate continues or increases then of course it makes no sense to try and time the good or bad days.

Of course, some will argue that a pure index fund market won't be priced correctly as it is active/day traders who continually buy and sell to set a proper market rate. If everyone buys and holds like Bitcoin then there is no true reflection of the companies value in the stock. Those that would go under otherwise may be buoyed or bought out just for their stock value. There are many opinion articles out there on the topic, but so far it hasn't been observed at scale (I think).

[1] https://en.wikipedia.org/wiki/Rule_of_72

[2] https://www.businessinsider.com/personal-finance/average-sto...