Well you don't HAVE to go on every community to see what every single community says about something though. You can just have the couple communities you follow and check those. Likely there will just be a couple of big communities for each topic, not dozens. What might happen even is that you have certain instances specializing in certain topics. You might have left wing and right political instances for example, so you'd just check the 1-2 instances you follow.
Each instance would effectively become analogous to the old time forums.
Like I said though, there is also the possibility of merging content from different instances into a single page.
Right, I don't know if anyone would want to post in a super giant community like reddit. Your post just gets lost in the void, content gets completely dumbed dumb, and no one knows anyone because there is too many users. This was a huge of appeal of the old time forums which got killed with reddit. I think the internet is going to fundamentally change.