I can't speak about kbin & Lemmy, however for Mastodon and Calckey there is a feature to export your user data and posts. You can then retain these for reference. Currently there is no ability to import them to a new instance -- the technical reason for this is beyond me, but I believe it's related to the UID of the posts as it is shared across the Fediverse.
People are working on solutions, and I have tested one which works well for Mastodon.
It's very important to have the ability to extract your own data and move
It's certainly a challenge. I run three instances on the #fediverse. Two small ones and a larger one with 450 users. I have a donation page; initially people were enthusiastic and I covered costs. Now, I have regular monthly donators but not enough to cover costs so I am subsidising it. I took the decision when I launched that it could happen and it's my problem.
I think there will be many instances will fall away in the coming months due to costs. Especially if you are thousands of users and associated costs.
We need to come up with a new funding model, where people appreciate you get nothing for nothing. All the large corporates sell your data as advertising for revenue. The greater public do not appreciate they are selling their soul.