My concern is that v4 may never be decommissioned. Sure, v6 could become predominant, but fundamentally there being two separate protocols running in parallel

My concern is that v4 may never be decommissioned. Sure, v6 could become predominant, but fundamentally there being two separate protocols running in parallel is a failed migration. On a much smaller scale it's analogous to migrating from mariadb to postgres, but the migration takes 50 years and you end up realistically maintaining both.