The majority of people will keep using the site like they always have, but I think the reputation and mindshare has taken a pretty big impact. Reddit is following Twitter's trajectory, and while people are still using Twitter it's sort of becoming a joke. Also Spez just can't stop talking about the protests and every time he does he says more and more stupid shit, which again, is completely following in Musk's footsteps.
I think both platforms are in a death spiral, even if they're maintaining a large userbase for now. I think almost everyone using Twitter, and now Reddit, are just waiting for a tipping point to occur elsewhere. Maybe if a few high profile users/musicians/news outlets jump ship, others will follow. I personally think the Fediverse is the next big thing, but it has work to do to get to where both Reddit and Twitter are now. Which is fine. It took both of those platforms many years to grow into what they are.
I still don't get it. If a r/pics mod goes to lemmy.ml and makes c/pics, I can go to lemmy.ca and make c/pics, and you can go to kbin and make m/pics. You're right that probably one of those pics communities is going to end up being the favorite but that doesn't mean the others can't post good relevant content. Also no one needs to "move communities" you can subscribe to every version of pics that that exists. I'm subscribed to multiple different communities of the same topic because each of them are going to have their own slant or take on the topic. Over time the content and comments will be what determines my favorite of them, not which is the biggest.
On the fediverse I think content is king, much more than anywhere else, simply because there can be so many versions of the same topic. The one that rises to the top will be content based, not based on server or who the owner is. I can create 50 communities, but can I post 50 communities worth of good content and foster 50 communities worth of good comments? I mean, maybe. But probably not.