If you can afford the downtime, pg_upgrade with the --link option is the fastest way to do the upgrade. It typically only takes minutes - the duration depends on the number of databases, not on their size.
If you are not planning to use pg_upgrade, but rather pg_dump/pg_restore (e.g. because your database is small or you don't want to migrate all databases in the instance/cluster), then there is indeed no need to use pg_basebackup.
