The problem with current incentives to upvote is just how many people are playing the game.  As long as everybody else is playing, the game seems worthwhile as the rewards are higher.

I wouldn't suggest removing the curation rewards completely, but the stake taken from the original post could be changed so that instead of 25% of the total, curators receive a limited amount.  That probably wouldn't eliminate the incentive to upvote, but you might find less people voting this way if there seems to be less people playing the game.