Image someday we have one million new posts a day, 10 whales with 40 votes each can only vote for 400 posts in total, and probably many votes will go to same posts, what will the other 999,600 authors think?

This is an interesting question. First, we have to take into account that community functionality will exists in the blockchain by then (hopefully). In practice, it means that those communities where the whales are, will be much more valuable than other communities.

I'd also guess that the amount of whales will reduce when the price of steem goes up and they sell part of their stake.