I don't see how this reduces returns for indexers in particular. Investing in an index means you get the market average. This may result in lower returns for th

I don't see how this reduces returns for indexers in particular. Investing in an index means you get the market average. This may result in lower returns for the total market and thus also for indexers but for some active investors to overperform thanks to this some other active investors will need to underperform. Winning that directional bet requires someone else to take the other side of the bet and lose. Total market returns are zero sum.