Individually powerful cards will shine through, however
Before we go any further, however, a quick decklist: Creatures (34) 4 Lovestruck Beast 4 Questing Beast 4 Yorvo, Lord of Garenbrig 4 Syr Faren, the Hengehammer 2 Nylea, Keen-Eyed 3 Kogla, the Titan Ape 2 Ashaya, Soul of the Wild 4 Tangled Florahedron 3 Old-Growth Troll 4 Kolvori, God of Kinship Lands (21) Spells (5) As proof of concept, heres a Mono-Green Legends deck that exactly meets the suggested nineteen legendary creatures
This is a frighteningly efficient card, and we'd be surprised if it didn't see a lot of play for some time to come
In formats like Vintage its an extra access to Ancestral Recall, and you can get a card like Demonic Tutor which will give you any card you want later
If the permanent that active player controls phase out at the end of the turn, it will phase in after the whole turn, at the next untap step of this player