I agree. The GU team recognized the issue, but their solution was incomplete. In Mythic, you only have to get 17 wins to get a legendary pack (compared to 20 wins in Ethereal Diamond). However, as you say, getting 15 wins in Mythic is no different from ED. They just need to rescale the 10-15 rewards to be slightly better than Diamond. An extra expansion rare pack here, an extra expansion epic pack there, etc.