I posted below.

The SP breakdown is 53% to 47% (roughly) for the Whales/non-whale accounts.

The reward payout is 75% to 25% author to curator.

Even if the Whales wanted, there is a point where they cannot post enough content to monopolize the reward pool where nothing is given out.

As you said, most Whales are passive investors leasing their SP out. Plus, they tend not to be buyers of STEEM since they have so much. The last price drop saw a lot of smaller accounts adding to their positions.