I migrated from a self-hosted mailcow-dockerized to the hosted version of Mailcow. It’s just because I don’t want to care about keeping everything up-to-date, secure and backup-ed. Now I have some stricter limits, but in the past I didn’t reach that limits and I doubt I will reach them ever. To migrate all emails, I used the online version of imapsync.
To answer the question why I didn’t migrate back to Fastmail (which I used previously): With Mailcow I have the possibility to create up to 20 different mailboxes instead of just one. Furthermore, Mailcow is hosted in Germany and since I am enthusiastic about the open source project, I would like to support the person behind it.