To answer your question, If there were no downvoting at all, then scamming the sytem will be so high that no one will be able to earn, because there would be millions of bot accounts sucking the reward pool dry.

The only "problem" with the "millions of bots" is because they get free delegated 15 steem from @steem.

Just kill the free delegation for newbz (which would essentially make steem "invite only" because you'd need a sponsor), and the "millions of bots" problem evaporates.