I think the main problem here is that there isn’t a really accessible explanation of federation and how these social platforms differ from the other, larger options. There is lots of great documentation for interested users to acquaint themselves with, but it would be beneficial to have a more ‘elevator pitch’ version that can get people moving through the signup process with more confidence. Even just a short message saying: “hey, choosing your instance isn’t as important as it looks right now, you’ll be able to freely use any other instance once you sign up” could go a long way towards making on-boarding much smoother. Once a user is in the system, they can learn what details they care about through osmosis for the most part.
I do think that having a default instance would help with streamlining the on-boarding process, but I don’t think that the idea aligns with the values of lemmy as a whole. It’s important to keep services decentralized in order to keep things free and open.
It would be nice if the instances could essentially federate themselves across instances. If two communities agree, they could both combine and show the posts from one another’s feeds, without sacrificing their autonomy. This way if you are subscribed to once instance’s community, you could see content from a much larger super community.