I really think this should happen. With the Christmas event, everyone was in with a chance of completing the event achievements. Unfortunately this time the vast majority of those who can even try and complete the achievements are those who can afford the "buy" them.
This is not fair in my opinion, as achievements are a really big part of the game for some players and they will have no way of getting them. They can buy the items from other players, but that in itself is going to cost EP (or real money).
You got the Christmas Event spot on in this regard. Even if you make the boxes cost lots of treats, you would also prolong the longevity of the event because players would need to grind out treats = boxes.
Please note, I have no interest in achievements myself that is why I have still not done any except the ones I gained naturally through game play. This suggestion is for the benefit of everyone, not me.
(If they are available for treats and I have just not found them, then please disregard)