I agree with OP that instances being closed any time is an issue that would need to be resolved fairly soon.
A new user currently has a choice for joining from a number of instances but there is no assurance to ongoing existence for them. Along with that, there is no way to transfer user accounts and data across instances, afaik. Having the option to transfer user accounts would help with the instance closing and eventually make the fediverse more stable.
If a user can transfer their accounts and data, there will be less hesitancy to join a new instance, and user accounts and data can be distributed across more instances. This can also work in such a way that if a subset of user data does not meet the criteria for another instance, then that subset of data is not migrated.
Another issue is with the presence of same community/magazine in multiple instances (let’s say [email protected] and [email protected]) which is frustrating for users since they need to track multiple communities for similar content and the same content is being copied to multiple communities. This should also be resolved by implementing account migration. We are already seeing that communities on certain instances are becoming the prevalent ones. This creates an incentive for the admin of those instances to not shut down. And if they did decide to shut down the instance, then the users can just migrate to another instance and the prevalent community will also get to keep all its data, just in the new instance.