With Reddit, I suspect it was mostly timing that can't be replicated unless something mind shattering and truly catastrophic were to happen. (Much more than what already has. Hey, it's possible Spez can fuck up even worse.) It bridged Web 1.0/2.0/3.0 (god I hate that term) and was able to facilitate mass user migrations from Digg and Fark along with a number of smaller communities that shut down or died off. Lemmy and Mastodon -do- have room to grow but I suspect people prefer "the one thing" to move to and unfortunately, there isn't that one other Reddit alternative, I'm happy enough to be here, but I'm not counting on this federated alternative to reach 10% of the total userbase that Reddit had.
Nothing really replaced LiveJournal. Nothing really replaced Tumblr. SmugMug, Flickr, Blogger were all functionally similar sites that just never got as far. I think we're going to see a bigger change in the style of the next social media site that reaches popularity more than we are going to see a functionally similar one replace Reddit and reach the same userlevels.
And I think we have the exact same problem with Twitter. Why would anyone want to move to another functionally similar microblogging site if Twitter still has more users? The problem we're seeing with post, mewe, etc... General people don't want to split themselves between several sites to keep in touch with everyone they know.
So they will just ignore what bothers them for as long as enough people remain on Twitter or Reddit.
I've been around long enough to eventually realize that no one community lasts. They all die. Whether it's due to attrition, incompetence, infighting or just those in charge giving up the pretense of professionalism and turning it into their little sandbox full of cat shit... Something new will always show up and it's a matter of time before something loses or reaches critical mass. However important that 'reaching critical mass' can be for some sites... it just doesn't seem like that happens as often anymore. TikTok being the last big example I can immediately think of. How many people do you see talking about their Kik, Snap or Telegram chats? "Everyone I know is on TikTok, I'll just go there instead." (Or Instagram/Facebook/Whatsapp.)
Whether it's general sites like Fark or specific forums like HardOCP/Forum screwing the pooch, the only thing I've been able to take away regardless of what happened: Sticking around always led to a more negative outcome. People only end up rewarding themselves with toxicity by choosing to remain with a site that has turned on its community and tried to tell others where they can or cannot go.
Those who insist on staying... I've stuck around on some sites/services for those one or two that I wanted to keep in touch with. It wasn't worth it in the end.
Cut loose. Enjoy the next spot. Tell people where you've gone but don't waste time with wondering if people are going to make it. If they care, they'll find their way.
This includes not trying to convince people to follow. If they need to be convinced, they likely won't have as good of an outcome as they expect from your sales pitch.