Thanks all, genuinely useful responses.
The opportunity cost framing is the right one. My actual goal is to beat the index, which is why I'm in individual stocks at all. Whether I have the skill to do that consistently is still an open question. I've matched the index over the past 5 years with meaningfully less volatility, which I take as a mild positive signal. Of course, four years is nowhere near enough to distinguish skill from luck.
All in all, I'm ready to take on that risk, as the rewards of even slight outperformance over decades are significant, although the opposite is also true. I have half my portfolio in individual picks and the other half in index ETFs as a hedge against my own stupidity.