And the relative values of those stocks will shift requiring rebalancing. You might be able to do that with new dollars for a while but hopefully, eventually, the swings are much more than new dollars and then what? Pay capital gains tax on sales to rebalance? Convince yourself the new random allocation is fine?

I don’t think this is correct. Gains historically accrue to a small number of companies in a given time window. If you buy all the grocery stores, you’re exposed only to sector risk, if you pick one or two, you’re also exposed to the risk those companies don’t contain the “winners”.

I don't know about the typical HN contributor but I personally lack the cash to but all the stocks in the S&P. There are 503 stocks tracked in the S&P 500 index. It would cost about 2.8 million USD to buy 100 shares (one board lot) of each if you were naive enough to weight your purchases that way. If you were to weight the stocks differently (eg. total market capitalization of each company) the amount would be higher.

Or, I can pick up 100 shares of an index ETF for a few thousand and have someone else do all the work for me including rebalancing and doing all the other required calculations (lot tracking and cost basis calculations etc.).