I guess over time the significant increase in number of daily downvotes will (at least for most authors) lead to nearly the same rewards as we now have.
The problem is that most people don't dare to downvote, anyway, because of fear of retaliation. (In the post My STEEM Vision. I wrote more about the problem.)
If downvotes will generate (additional) curation rewards in future I may create a new (peaceful, not retaliating) account just to downvote it regularly. :-)
Yes, that's true, especially for smaller SP holders, but I guess larger accounts will build/use kind of automatic downvote services (maybe targeted at 'overpaid' authors on the trending page) to maximize their own rewards.
Maybe some people will even create an extra (unrelated) account for not having to risk their main account being targeted.
Actually, I had such an extra account just for flagging whales ... but it had not enough STEEM power to make any difference. :)