Continuing to mutate on top of one or several Greathorns can really help a player get ahead on mana while also triggering other mutate abilities that have been combined with it during the game
For another, the decks would stay a lot more fun to play as new sets come out with room to incorporate other cards
If it later becomes a creature and damage is dealt to it, that damage won't remove any loyalty counters
Heroes Reunion probably seems like an odd card to be slotted higher than Call of the Conclave, but hear me out: Call of the Conclave is Watchwolf plus
Sets 1-4 (The First Chapter, Rise of the Floodborn, Into the Inklands, and Ursula's Return) already left Standard in the game's first rotation, back in September 2025
Etali, Primal Storm is a six mana 6/6 Dinosaur with great stats even without its additional effect