Jack Bogle founded Vanguard, invented the index fund, and spent fifty years telling anyone who'd listen that low-cost passive investing is the best deal in finance. He was right about a lot of things. He was also wrong about one or two — and the things he was wrong about are what most modern indexers still trip on.

What he was right about

1. Costs compound, brutally. A 2% management fee doesn't sound bad until you compound it across 40 years and notice it's eaten 35% of your terminal wealth. Bogle's central insight was that the expected return of active management, net of fees, is approximately the index minus fees. He was right. The data has only gotten more punishing.

2. Market timing is a tax. Selling at the bottom and buying at the top is the single most expensive habit retail investors have. Bogle's prescription — don't — sounds glib until you look at the dollar-weighted vs time-weighted return gap. Real investor returns trail the funds they hold by 1–3% a year because of timing.

3. Diversification across "the market," not "stocks I like." A total-market index has hundreds or thousands of holdings. Concentrated bets have to clear an enormous evidence bar to be worth the variance. Almost none do.

What he was probably wrong about

Home-country bias. Bogle was famously skeptical of international diversification. His argument: most US companies have global revenue, so an S&P 500 investor already gets international exposure indirectly.

The math doesn't support it. International diversification reduces variance for a given expected return — that's the free lunch in modern portfolio theory. Whether US-exceptionalism continues for the next 40 years is a bet, and concentrating in one country is a way to take that bet whether you meant to or not.

What this means for you

For most builders with most portfolios, the answer is still mostly Bogle: total-world index, low fees, automatic contributions, don't touch it. The marginal hour spent fiddling with allocation almost certainly has a lower expected value than the marginal hour spent on your career or your business.

But: total-world, not just S&P 500. The home bias is a free decision to leave money on the table. Fix it once and move on.