he be able to assign a massive amount of the reward pool with noone to gainsay him.
Moderators would downvote overpaid content, this user would be wasting his power if he was upvoting shitty content. He'd better off rewarding real content that's not going to be downvoted.
Allocating a good portion of the reward pool to yourself is something whales could do today so why don't they? There is multiple reasons for that but the primary one is that they are going to be downvoted if they do. So it really is no different to how the system works now.