Lemmy.ml is not only a massive instance, it is also the original and core lemmy instance. Widespread defederation would be like a nuclear bomb to the lemmy platform.
Some people have developed alternatives in the threadiverse like kbin or piefed. If lemmy.ml is truly too far aflight for users to tolerate, it seems likely that alternative platforms will fill in the gaps. For now, lemmy is still a thriving and growing platform.