I have not heard about a single whale being punished, lost their power and forced to leave.

If they behave as you imply (I don't know anything about it), they will be punished quite severely - their Steem shares will lose value. It's hard to imagine a more appropriate punishment.

What would you suggest instead? Some sort of committee trusted to make fair choices?

If such a committee does not have skin in the game, sooner or later it will start abusing its power - we know this for sure, that's the human nature.

If it does have skin in the game (by owning Steem shares) - we end up with what we have now: whales having decisive upvote power.

Steem is not perfect and it's not meant to be 100% fair. But it's the best incentive structure we can come up with, at least on this planet.