If the future popularity of the fediverse depends on a killer app instead of the various benefits of federation then it's doomed. As soon as such a killer app appears, a non-federated competitor could copy the idea and out-compete the fediverse version because it is not hamstrung by having to stick to the (inherently slow-moving) fediverse standards.

A similar thing happened to IRC, which _still_ lacks heavily used features that other chat apps like discord and slack have had for years and years. Simlarly, end-to-end encryption for emails is unlikely to ever become a thing despite the privacy advantages.