Demi Garrett left, but Yusuf Zuck also gained a lot of information from Demi Garrett.
First, the universe is a multiverse, with many universes, countless in number. Even for someone like Demi Garrett, only a small part has been reached, with a large part still unknown.
Second, within those unknown universes, there are also super strong beings called Time-space Hunters. They are not in the Eternity Realm but can still wield the Eternal Life Force.
Third, the Source of the Universe from the Transit Realm, which Yusuf obtained earlier, is indeed not useless; he just hasn't been able to truly utilize it yet. Once he reaches the rank of Dao Sovereign, he'll be able to use a tiny bit of it; achieving the Eternity Realm might allow him to use even more.
Bear in mind, this is the Transit Realm, and since he obtained the Source of the Universe from here, logically, everything within the Transit Realm should be controllable by him.
Yet, he currently can't control anything.
