Instead of one community becoming completely dominant on a topic, there’s another one close on its heels should anything happen to the first.
And if I subscribe to both, who cares which a particular post comes from? Just scroll down the feed, read a post if it looks interesting, ignore if it doesn’t. Which community it originates from doesn’t matter.
I wonder if the people who push for one community per topic across all the Fediverse are just extreme tidiness types who get a kick out of seeing everything in orderly little boxes. Trying to Marie Kondo a decentralized internet forum, that way lies madness.