In an efficient market players will be priced on expected dividend returns. As the index grows it will get more efficient so get players that already return dividends or are likely to return dividends in the future (this can be hard to predict of course).
Yep. And another consequence of efficiency is that when someone wins, someone begins to lose. Atm a massive proportion are winning, and a small proportion losing. This gap will decrease over time and it will become harder to win. Long way off yet though it's easy pickings for a while.