Even the absolute worst luck gives you perfectly acceptable returns on 30-35 year timelines. Multiple orders of magnitude better than a savings account, for sure.
Time and time again the best approach for highly compensated people who don't want to actively manage their finances is to put a chunk of every paycheck into a broadly diversified range of index funds and forget that it exists. In the US this means a) getting your company 401(k) match no matter what; b) putting the rest into a Roth IRA as cash flow/debt service allows; c) putting the rest into the remainder of your 401(k) as cash flow/debt service allows; d) putting the rest into a non-tax-advantaged brokerage account that basically mimics your 401(k) but probably has better fund options.
Meaning if you were planning on retiring during one of the long sideways or downturns your required "time in the market" may have just been extended by a decade. The impact of that depends on the unique time constraints of the individual and how well they addressed risk exposure.
Add to that, most economists don't think the long-term market returns will approach the past returns. Most now think 6% is reasonable, when the past was 10%-12%. This extends the horizon further unless you are willing to increase risk exposure by selecting higher growth stocks.
Your comment was that equity investing can be disastrous if you hit the timeline wrong, and my only point was that unless you're speculating or are only in the market for 5-10 years, that has never happened. We have no way of knowing if that will happen in the future or not, but there's no reason to believe that we're going to see decades of middling or negative growth. There's really no other option if you want to have a real retirement unless you plan to just rely on whatever government assistance you can get.
>that has never happened
This is the part that I disagree with. Talk with anyone who planned on retiring around 2008-ish. As a hypothetical, if the market is the proxy measure they lost around 5-8 years of retirement because they had to have "more time in the market" to re-coup losses. If they actually retired and were drawing down their money, it's even worse. Granted, the hypothetical is biased because someone of retirement age shouldn't have that much market exposure, but the point still stands that, while true at the population level, waiting for the market to recover doesn't always work out well at the individual level. I tend to think the people who tout long-term averages of market returns tend to ignore the long periods of sideways or downward movement that may align with an individual's specific circumstances.
