IMO, PeerTube will never become popular due to its decentralized nature.
It doesn't have to be popular in order to be successful, or worthwhile.
As a consultant I have frequently been in a situation where each member of 'my' team was only outnumbered about 6:1 by the customer. That can be directly managed. As a business owner, I can afford to have a personal relationship with a few percent of my customers. I can cherry pick. As a medium application developer, each of my employees might represent 1000 users, and even zone defense starts to crumble. I have to start treating them as numbers, and that causes new types of problems.
I see the sorts of compromises we often have to make at hundreds of thousands of users, and I'm grateful that most of the verticals I'm interested in wouldn't ever have 600 million users, let alone ten times that much. There are verticals where form and function are tightly aligned, and I think those can scale much higher. Torrents and Discord, off the top of my head, I think resemble that remark. Infrastructure like nginx or Consul, maybe. Linux... well, it's 5 million lines of code so it might be 'failing' in that manner.
Reddit, on the other hand... I'm glad I'm not responsible for Reddit because I'd probably hate myself half as much as my users claim to.
Some comedian had a bit years ago about how he wanted to be 'Bruce Campbell famous'. This is a guy who is a god in a niche. If he wants his ego stoked, he just needs to be in the right room, and he can make that room happen just by asking for it. Meanwhile he can go out to dinner at a nice restaurant and there is a reasonable expectation that he might get to eat in peace. I think I could deal with that level of success.