They will bind it to the current sub imo. The profit for Blizzard is the growth of the playerbase. There will be additional subs through Vanilla-only Players. They will also achieve to bring some of the new subs back to retail. At the end they just want the community to stay within their bnet client. That was also the only reason to bring destiny 2 to bnet. They just didn't want to loose part of the overwatch community.