But all the of the follower/following data is available via CSV export any time, too. And there's obviously an open API as well.
And honestly, you could also just run your own server if you're that concerned.
To be fair, that's an improvement over, say, Twitter, where they'd just be gone.
I understand that I can do this by hosting my own instance - that's a good enough solution (for me).
Most Mastodon instances are community run, and when they hit some rough straits in paying for their hosting some of them will have a (metaphorical) bake sale encouraging users to contribute to the hosting fund Patreon or will ask around if someone wants to take over the admin hot seat for a while.
For those concerned about the longevity of specific instances that's generally the answer: ask the community how the admin pays for things, look for their Patreon, consider chipping in what you can each month.