The incentive is a higher multiplier for their actual rewards. If they go lower with the cap (less greedy) they get more.

The whales are voters in the system, not authors, so the proposal only applies indirectly in terms of curation rewards for them.

But in case a big whale writes an article, they may choose to cap it intentionally to avoid extreme upvotes. Say if the Steemit creators want to make a post, they can put the limit to 500-1000 (enough to be "trending") but not more than that (so that rewards could go to smaller authors).