This post assumes you have a basic understanding of what Lemmy and Kbin are. Which, in turn, assumes you understand at least the concept of federation.
The Issue Currently, the way Lemmy/Kbin (which I will refer to simply as “Lemmy” for the rest of this article) work like this:
Cool, so our user can access content from both instances. They go to Instance #1 to talk about transporation, and Instance #2 to talk about animals.
While OP has a point regarding a potential problem, the term ‘fundamentally broken’ is an absolute overstatement. At most, it could be stated that Lemmy/kbin might not be feature complete yet. And i expect, given the crowd sourced nature of it, that it will dynamically evolve into our users combined needs. No need to abandon it!
Now about the actual point: as others have mentioned, the way Lemmy handles multiple subs on same topic on different instances is good. The communities are easily differentiable via the instance prefix, and if i want both, i can simple subscribe to both, and will see the posts from both in my feed.
I don’t like the tag idea, because such global consensus about what e.g. a horse is might work for a horse, but not for other terms like ‘politics’, which would depend on the jnstance. If i see a politics sub on an italian instance, i expect it to be mostly about italian politics and related world politics, which is good.