The nature of Human being is selfish and greedy.

Not everyone. There are many good, unselfish/selfless people.

If we remove the rewards, what will happen in the steemit community.

There is only one way to find out. :)
The leader of the Steemit should really try that. At least for a few weeks. I would see what would happen. Probably the number of the posts would decrease close to zero. I say close to zero, because I saw some people, who published at least some of their posts with the "decline payout" setting, so a few people would maybe stay for posting even without the rewards.