That's a community/mindset issue, not a fediverse issue I think. It was useful because that's where the people went, so if the people went somewhere else, the niche communities would thrive there just the same I'd think.
Over the last 10 or so years this centralized mindset has really taken a big hold. Before sites like Reddit, Twitter, FB, etc. existed, the internet was basically all niche communities that all had their own space.
I think especially for niche communities the fediverse could be a fantastic opportunity.