First, this game is not created purely to serve the type of person that likes to play it 16 hours a day 7 days in the week. The game is created with the average gamer in mind and the average gamer is far from what you'd call a hardcore gamer. There are millions of gamers and most of them probably have full-time jobs, go to school, have kids and/or have other plans. Most of them can't or won't take holidays for the release of a game. It wouldn't even be ethical to create a game for the hardcore gamer and Blizzard even communicates this in some tips. They might want your money, but they don't want you to waste your life on a game that only has entertainment as main goal.
So when you create something for thekind of gamer who has a few hours on three evenings in a week and a bit more time in the weekend, you don't want to give them the feeling they are missing out big-time. Without the gating it would be exactly that. Only the top % of players who level in no-time and did everything there was to do in the first three days of the first week might manage to keep up and not feel like they are being forced to keep playing when they can't afford it.
It may be a bummer for you, but you are one among milions and belong to a part of the target audience that does not have the main focus for Blizzard.