While this is a valid point, there are some ways to deal with it. For example on reddit, if you go to the profile of an user and downvote every post, it does n

While this is a valid point, there are some ways to deal with it. For example on reddit, if you go to the profile of an user and downvote every post, it does not count the downvotes. You would have to open up the post and then downvote. (We could probably also detect if it is a "natural" visit or not) So you can make the process very time consuming at least. Sure there are bots but these can also be detected. So yes, it can never be fully stopped but on the other hand you can do similar manipulation without downvotes so I feel it is more of an parallel issue. You can still have a group voting each others posts up or straight up buy votes.