If I were the only person to make a steemit post one day, or week, and I was the only person to vote and I upvoted it, I wouldn't earn the entire reward fund and if nobody else was posting or voting, its likely the coin would have died and therefore the SBD/Steem ration would be so low (due to a dead coin and the steem price dying) that my vote would be worth nothing... However I would earn 100% of the rewards rewarded for that set period of time.

It doesn't matter what all the other posts are voted to right now, the predicted value of vote is based on the amount of steem in the reward fund at the time of the vote and again at payout, along with a 3 days average for the price of steem... and so much more.

In a situation where 1 person owns 51% of all of the steem, yes it would be way better to flag their vote but its not like flagging prevents you from posting.... For instance if they post 10 things a day and upvote all to 100, I could post 1 thing, upvote it to 100 then flag his 10 posts at once (which will use around 19.92% of your VP) and since the value of his vote will decrease you could still coordinate a flag to drop the posts to 0 as their VP will decrease as well,
If they also flag then their VP will drop too, if they wait between posts then voting 10 times can use more VP than voting 11 times at 100%.