There is one thing an index fund doesn't give you: food. So it seems logical to compare it to conventional greenhouses instead of comparing it to financial investments.
Of course, if all tomato growers closed shop and put their money in index funds, we wouldn't have tomatoes any more. But that would make the price of tomatoes shoot up and the return on index funds crash down, making tomatoes a better investment than index funds.
Unless, of course, angry hordes murder you because they tend to dislike people who watch famines as a normal market mechanism and suggest diversifying into cakes.