People solve this problem via admin teams, and yes they do tend to share privileged access to infra as their backup plan.
When the worst does happen and an instance shuts down, federation itself is a pretty strong escape hatch. There's no account migration yet, but you CAN recreate a new account elsewhere and resign up to most of the same communities. Posts/comments on communities that were lost on the server have many posts/comments cached elsewhere. It's likely that activists could recover much content if they put the effort in.
That's it to say that loss of an instance isn't a large inconvenience, but people already have reasonable tools to handle it. Maybe over time the capabilities will improve here, but right now the focus is on performance and mod tools, both of which are absolutely essential to weathering the current growth spike.