I don't think the risk is "absurd". Or, at least it's no different than the risk they ask any investor to take by charging them 1% of their portfolio for it to be "actively managed".
Plus, they are being compensated. I'm offering 90% of the returns above the index :-)
Active funds ask investors to accept 100% of the downside and get taxed on the upside. Actually it’s worse: they’re taxed on both the up and down sides.
If this is a terrible deal for fund managers then virtually by definition actively-managed funds are a terrible deal for investors.
The risk your (completely hypothetical) offer would involve is reputational. There's no reason for a funds manager to ever risk their reputation on your stunt since they are constantly risking money and reputation in the ways that they control. Indeed, one could almost certainly put together a bet similar to yours using derivatives and have the potential upsides and downsides without the reputational damage.
Of course, if you offered your bet to all comers and gave significant publicity, unknown "funds managers" would be happy to take you up, though they might well default if they lost.
Plus, they are being compensated. I'm offering 90% of the returns above the index :-)