Bets aren't profitable on longer-term questions. The opportunity cost versus sitting in an index fund would be a nominal ~11% per year. That means for longer term timeframes, even with a betting edge, your opportunity cost increases exponentially. This is a problem for the usefulness of betting markets.

When will there be a betting market that does what I want -- your bets and winnings are paid out in some way that is e.g. Vanguard index fund pegged, or something roughly equivalent. This would then make it rational for great forecasters to do stuff in betting markets on long-term questions, and not be losing money the whole time. Realistically it's unlikely to be quite as high return/risk as sitting in the index in a broker's account. But I'd accept something that comes close.