A bridge is the first step in the “Embrace, Extend, Extinguish” pattern. A Fediverse only works when no one node can dominate all the others. It’s why, even though Meta’s Threads platform was/is actually Federated (in that it uses ActivityPub), everyone defederated from them because it’s a poison pill for the whole network.

For more info on how that eventually happens, this article gives some past examples.