We've had Usenet, Forum, IIM, MySpace, Facebook, Reddit, etc. etc. They've all kind of started out fragmented but over time people naturally built up their communities and figured out what worked.
Kbin and Lemmy aren't that much different from Usenet or Forums, its just the terminology that is messing people up.
On Usenet communities ended up getting split up because people just really liked to spin off sub groups so you start with comp.technology, then comp.technology.linux, comp.technology.linux.ubuntu etc etc etc.
Forums were always fragmented communities. I have ForumA with these threads and ForumB with these threads, ForumB will never see the posts from ForumA unless they go to that website to see them (and vice versa).
In the Fediverse, sure communities might end up fragmented because each instance has a @technology BUT the benefit is I am on InstanceA and you are on InstanceB and as long as we are federated you can see all of the content from my instance and i can see all the content from yours.
Now, that all being said... One feature I am pushing for to get added to kbin is something along the lines of a multi-subreddit. That way you can set up technology@lemmy.world technology@beehaw.org technology@kbin.social etc to be in this multi-subreddit so as a user you will only see posts from @technology Users don't want to mess with 50 different tech communities but if we had a multi-subreddit feature that blends them all together so it only appeared as @technology I think it would win a lot more people over.
I spun up my own kbin instance so I can hopefully start helping with the development of features (and to lessen the load for other instances). The two features I'm hankering for at the moment are API support so I can write some content aggregator bots and the multi-subbredit feature.
Anyways that is my rant? tedtalk™? Idk, hopefully all of that made sense to someone out there.