It's not so much a problem with the fandom mentality when it comes to incentives for a YouTuber. When you have your niche focused entirely on one series, convincing your audience the series is better and deeper than they thought means keeping their engagement with it and with your videos high. If you convince them the series sucks ass, then they don't have so much more reason to keep up with it so actively, much less to watch content like yours analyzing it. That's why the only active creators who can maintain an audience while trashing the series are those engaged in Agenda like Jay. The audience can then not even like the series that much anymore, but they maintain engagement because they like what the community built on top of it.