Pretty much this. Many players take time out of their normal day to play a new expansion, so if Blizz don't gate the content, people are gonna be done in a week or 2, since people will play 10-12 hours a day.
Some would say, that there are few people who do this, but its actually a huge part of the playerbase atm. So we need gating or else the game would simply become boring very quickly.
From what i can see from 7.1, they are putting in more content then they have ever done before, so if they fail with Legion, it is not because of the devs, but because the playerbase demands something impossible.