You’re correct that you can’t just go to one community on one instance and see every new technology discussion that is taking place on Lemmy, but you CAN subscribe to all of the technology-related communities on different instances and scrolling through posts of communities you’re subscribed to will show you all the discussions you want to see.
Ideally yes, but for a more niche community, federation is slower and posts don’t get pushed through. I can see a lot of confusion when someone creates a niche sub because they thought it didn’t exist, when in reality it does, and they were just the first to search for it from their server.
I don’t care for them myself, but unless they’re causing problems outside for everyone else, why defederate?
If there are communities you don’t want to see, you can block them as they appear.