I think your sportsbook example is very interesting but the odds for skilled sports bettors are probably a lot better than they are for a casino player who will lose all his money assuming an infinite time horizon. I´m convinced that wagershare is worse than sharing losses.
Has it ever occured to anyone to ask the Wizard of Odds to look into this wagershare isseu? This is a matter of casino maths after all, imho and I think we may be overlooking something that a mathematician may be able to pick up at first glance.