It's a migration for the future, not a migration of your past content - understandable because that past content likely has references from other instances and pushing those changes across would be a heavy operation, unreliable, and probably fraught with potential security problems.
I believe for retaining content it's pretty much retaining it for yourself by downloading an archive, though some instances may allow uploading that. There's also at least one browser-based web app that lets you open and view the content of that archive - otherwise it's kind of useless.