It's not like that box has been firmly closed until now. Every time someone stands to profit from one outcome over another they already have an incentive to influence the outcome, prediction market existing or not. And stock markets already act as a sort of prediction basket about future events that will influence the trajectory of a company (e.g. the outcome of trade negotiations, wars, court decisions, elections, the health of their CEO etc. etc.)
The upside of prediction markets is that it incentives people with information to make their honest estimates legible to society. E.g. an opinion piece in a newspaper has little skin in the game, other than the author's reputation.