Other insiders and people who think they know better?
For large/complex enough bets, you won't have a single group of people who know how things truly are - you'll have multiple groups of people who each have (or believe they have) a critical piece of the puzzle, possibly by proxy, that makes them think they have a crucial perspective - and those points of view will conflict, giving each such participant a reason to bet. Taken in aggregate, you get a system that pulls in all those perspectives and distills a single signal out of them.
I don't think we have the same understanding of "insider". There are dozens to several hundred insiders at any one time. There aren't enough for them to reliably trade against ones another. To day nothing of how this system would fail when a company decided to raise capital. Like, is the CTO going to trade with the CEO because they have different views on the company? When they need to raise capital, are they going to get it from an accountant at E&Y who's auditing their books?
For large/complex enough bets, you won't have a single group of people who know how things truly are - you'll have multiple groups of people who each have (or believe they have) a critical piece of the puzzle, possibly by proxy, that makes them think they have a crucial perspective - and those points of view will conflict, giving each such participant a reason to bet. Taken in aggregate, you get a system that pulls in all those perspectives and distills a single signal out of them.