[spoiler]While Retsupurae might fall into that category I don't think they are much of a problem compared to the other hacks out there, especially since the videos they riff on are really terrible, don't have a lot of views, and don't have a lot of effort put into them. So they may actually be helping the videos they riff unintentionally despite making fun of them since their videos get more view, though they may get an insurge of comments making fun of them too. The other people are stealing though and riding off the coat tails of more successful Youtubers.
Not to mention this problem affects game trailers, cartoons and anime too, every time you search for something like that there's about 5 react videos on the first page. So yeah it's becoming a huge problem now.[/spoiler]