This is nonsense. The fediverse isn't cryptocurrency. Having 51% of the fediverse doesn't give you any more control than having 1%. If your instance(s) implement a feature that the rest of the fediverse doesn't like, they can defederate.

Other instances either react by defederating, but because they only have 49 percent, due to network effects, they get extinct

If 49% of the fediverse defederates from the other 51%, it is now 100% of a new, smaller fediverse. You can't just claim that "network effects" will cause them to go extinct. Whether those instances have enough userbase to sustain a cohesive network depends on the actual number of instances/users. And the fediverse has sustained itself for over a decade with less than the current ~2 million accts and most of that time it had substantially less than 1 active accts.