What if curation rewards were inversely proportional somehow to the Steem Power of the curated author? Then it'd have more of a focus on lifting up fresh talent than grinding whales.
I think that would be problematic too. Whales are incentivized to stick around and create quality content because of the income. If there's diminishing returns on larger number of followers, their earnings would plateau and they'd probably be happier on another platform where they don't have that issue.
Diminishing returns on interactions don't necessarily have to penalize whales. You could still have full returns to the whale, with diminished returns to the curator. It's not ideal, but it's food for thought.