well I can talk to a service provider who has migrated, a few have - but we simply haven't done any huge migrations yet. Most of that is because big installations usually move slow - nobody wants to be first. Not that upgrading is a big deal, it is the same as an upgrade of course. In a way, once ownCloud does a new release customers should be more worried about that than about upgrading to Nextcloud... But yeah, how to explain that 😉
We will see what Nextcloud 11 brings, people might find compelling reasons to upgrade at that point.