If the instance defederates, users do not have an option to interact with the defederated instance anymore. The only choice you have then is to join an instance where the moderation policy agrees with your values or host your own instance. If you just want to see if defederation was the right idea, you could just visit those instances directly and look at their local timeline.
Sync between devices. I only read RSS on one device so I don't need it either. Besides if you don't think a service is useful to you why do you host it?
This didn't happen. In fact Google was the one who fell behind development when the protocol moved on and deprecated unencrypted connections for example. People just don't make it a priority to use XMPP instead of the walled gardens they are using now.
If the instance defederates, users do not have an option to interact with the defederated instance anymore. The only choice you have then is to join an instance where the moderation policy agrees with your values or host your own instance. If you just want to see if defederation was the right idea, you could just visit those instances directly and look at their local timeline.