I also think the premium edition will stick around for a long time.
Some people are waiting for the final release/the first videos/reviews of the game and then decide if they want to buy it. Would be a bit unfair for those people to deny them the extras..
I'm also interested how they going to add those 3 animals for people who bought the basegame, and then decide they want those 3 animals as well.
(I don't like the 'You can only get this extra character/animal, when you pre-order', like some developers do)