I think you put too much stock into the ddos explanation. I’m not saying it’s not a thing, but there are bigger issues that it seems won’t be resolved before Lemmy 1.0
Regarding your first question, tf is my home but I have created communities on different instances. Some instances because the community fits the instance, some because I like the URL. I figure it’s best to decentralise as much as possible.
As to your last point, I think some of what the admins at world have done is tremendous and I celebrate their commitment to world, but there’s limitations to the software, as they well know and ultimately this is a decentralised platform of which loads of little instances are supposed to make up the larger whole.