You make a very good argument against prediction markets in which the stake has actual material value, but what about markets where the only stake is your reputation? My understanding of the Google platforms is that there were prize rewards for top predictors but largely the rewards were social, which, to me, implies they were not taken particularly seriously. An ideal platform might be one where there is no material incentive but users are instead interested in genuine hypothesizing about future events. Obviously there must be a genuine stake involved, but as we see with Manifold this can be done with 'play money'.

Would be very interested to see further discussion about this.