It seems to me that making this an instant was just a bad idea, but the card was a bad idea anyhow, especially at the time, where they were experimenting with mana and making tons of mana was just par for the course, which made the whole thing much worse
At the same time, she made it clear that the Play Design team are respectful of Amulet Titan and other long-standing archetypes, and the general desire of most Modern players to have established decks remain playable in this Eternal format: We take bans against historically iconic archetypes very seriously and dont want to outright kill the deck
Dethrone is fine, and it gives you something useful to do with the guy
Butstill, you get that extra combat phase, which you might just be able to take advantage of
Whenever you, or a permanent you control, becomes the target of an ability or spell an opponent controls, you may draw a card