You're thinking about this from a pirate's point of view: "they can't sue all of us", and that's entirely correct. But it's also not what I'm worried about. People who know they're breaking the law and don't give a shit will take steps to protect themselves from legal prosecution. People who just see "oh hey free movies" will wind up using insecure services that get them lawsuits. This is what happened with people who used Gnutella and KaZaA back in the 2000s, or people who pirated fake porn from Strike 3 Holdings, or people who use Popcorn Time now. The more accessible an infringing service is, the more likely people will get legal nastygrams in the mail.

Imagine you're an average Internet user, you follow, say, a YouTuber that reviews movies. They say that their latest review isn't on YouTube, but on their website. You go there, watch the review, laugh, and move on with your life. But then you get a nastygram in the mail a week later. Turns out that particular YouTuber didn't have great copyright hygiene, and used an unlicensed music clip. Them moving it to PeerTube means that instead of their video getting taken down, the copyright holder has to sue the swarm - and they do. You get caught in that dragnet, and now you have a nice fat $10,000+ settlement you have to pay for watching a video, because PeerTube puts you in the chain of copyright liability.