> However some users will not be motivated by pure economics

This is true. The assumption is that a user who holds a lot of stake is likely to have either invested (which normally implies some level of understanding of long term economic incentives) or received the stake as a reward for posting good content or curating (which implies that the user is relatively constructive, intelligent, sociable and not a complete troll). Although this isn't exact science and there will certainly be many stakeholders who will use downvoting unwisely in spite of the fact it reflects negatively on their investment. But it is expected (hoped?) that such stakeholders will not be collectively holding a large amount of stake.