Earlier quoted context omitted.
If active investment were negative-sum in aggregate then society would be better off if there were no active investors at all and everybody would invest in passive index funds instead. Unfortunately you need active investors to keep the market honest, without active investors index funds wouldn't work! Index funds are needed so investment funds don't overcharge their customers, and investment funds are needed to keep…
I think you and your interlocutors are talking past each other. It's true (and fashionable to point out) that active investment is what makes price discovery, and thus the markets themselves, work. But that doesn't make it a good idea to invest in hedge funds or any other active investment vehicles. Just as Buffett describes earlier in the letter w/r/t the insurance business, active investors can compete the returns…
Worldwide equities are 65 trillion or so. Can you have a functioning stock market if 1T of that is actively managed by prop traders and the remaining 64T is in passive index funds? I'm not so sure.