But we want to make money today into money tomorrow in the most efficient way that involves the least amount of worry. After all, we have specialised in some other area of human activity, and are not good at this area. So we should pick an investment that requires no upkeep, no worry, and good returns. This rules out real estate entirely, and the last criterion rules out letting money sit there.

This is pretty obviously wrong. Minimizing worry usually involves investments like T-bills or money market funds, which are notable for high security and rock-bottom returns. The trick is to make the reward/worry ratio worthwhile, not to take worry minimization as a goal in and of itself.

I'm all for index funds - my own portfolio is a collection of 4 index funds. But there's a place for investments in other things. I've seen several folks do the "Buy a house near my kid's university, have them do all the tenant-management crap, then either foist it off on a property manager or sell it afterwards" investment plan, for example, and it usually works well.