@pinkdogfan

Sure, servers can choose to block other servers.

Server owners can run in a completely isolated mode if they want. (It even notes this as a possibility in the Masto video.)

But once people are used to being able to talk to different servers, it becomes difficult to put the genie back in the bottle.

Google hasn't dared stop Gmail working with other email providers for example.

You're right that there are always dangers, but a federated structure minimises them.