Everything in this long thread makes a lot of sense and without disagreeing with any of it I think the piece missing is return on equity of the underlying businesses. Over a long enough period that is the key determinator (is that a word?) of your return.

12% over say 60 years is 1000x your money. If valuations go down 75% over that same period you end up with 250x your money.

6% over 60 years is 33x your money. If you bought cheap and valuations go up 5x over that period (say from pe10 to pe50) you make 165x your money.

There are a number of complications to this simplified example granted, and I don’t own any index funds myself but high or decent underlying returns on capital I think would be the best argument for long term close your eyes and jump or DCA retirement investing for your kids.