100 people spread across 80 communities isn't useful, but neither is 100,000 in 1. It just creates an overwhelming amount of noise. Comments get filled with jackasses jockeying for attention, and niche posts get buried in floods of other posts.
Just because people have become used to it doesn't mean it's been good. It just means people have been blind to what they've lost, all because we have biases toward big numbers.
Niche interests won't be spread out. That's not how they worked before Reddit, and it's not how they'll work after it. Big popular groups will splinter, and it'll be a good thing.