Calling them dead or about to be killed, is a wide stretch. All of them do got some dips and people leaving these sites, but I don't see a big rush away, as the content is still there. With YouTube you don't have an alternative. With Twitter some went to Mastodon, some to Bluesky and some to Trump Twitter. Reddit is still doing good and Facebook... The whole concept is not doing well.
For all it's important, including fediverse is the content. Let just focus on Lemmy and Reddit. With Reddit you did have communities splitting of abusive mods. With Lemmy you suddenly have some general purpose communities on different instances and you need to figure out which one is worth following. Some communities quickly die off and they go back to reddit, as there are more users for that community. This alone is hurting whole Lemmy. When you then have somewhat active communities across different Instances, because they think, their instance is better or the other instances sucks, then you split the community more.
To survive you need content and active communities. Reddit still has a lot of that and Lemmy is trying to get more, but their design makes it hard to keep it together. Lemmy has just a few options to survive long term. Get more people and content, so the fragmentation doesn't play a huge role. Change the design, to prevent fragmentation or the users stop fighting and stop fragmenting the content and try to agree on one community and the others get deleted.
Stuff like Mastodon/Bluesky don't have such a huge problem, as it's about a single person and not a community.