>>I believe that different communities might have different definitions of "toxicity"
Sure. The problem is that despite the decentralized nature of those communities, they still don't exist in a vacuum. Their various beliefs and behaviors (that the community itself may not consider toxic) inevitably leak out and cause problems for society at large.
The incel community, for example, worships Elliott Rodgers, the guy who went on an insane shooting rampage because sorority girls wouldn't have sex with him. How long until another Rodgers appears? Why should we allow such breeding grounds to exist?
Some leftist communities worship Marx, Lenin, Stalin, Trotsky, Mao, Ho Chi Minh, Pol Pot and others, whose actions led to murder of tens of millions of people. Why should we allow any of them to exist?
Decentralization simply leads to the creation of echo-chambers, except with no one to keep the toxicity in check.