My personal take on it is that "immersion" is too often assumed to be some sacred cow that must be preserved at all costs. I see immersion as something that, while awesome at first, turns stale after a while. It has a limited shelf life.
Why? Because the fun offered by a sense of immersion is the sense of mystery, exploration and discovery. Once you've explored the content and spent time immersed in it until you are familiar with it, the sense of discovery and exploration is finished. At this point most players quickly lose interest in immersing themselves in the environment. That is boring. Players want to get to the meaty bits of the game, and forcing us to jump through proverbial hoops in order to maintain that sense of immersion goes from being awesome and interesting to tedious and boring.
Thus the desire to maintain a sense of immersion is misguided - and this was the fault with WoD flying - the flawed thinking that assumed that forcing players to remain immersed in expended content was a good idea. It's an easy enough trap to fall into and it's not that the thinking is completely ridiculous, but they did miss a trick. Immersion in new content is great. And it makes sense that players miss that awesome feeling that comes from experiencing new content. But the belief that immersion can maintain that feeling is a fallacy. The feeling is going to fade because the players have experienced the content, and no amount of forcing them to remain immersed is going to get that feeling back. Worse yet, forcing the immersion ends up detracting from the experience because it becomes tedious.
While flying too soon certain can ruin the positive immersive experience with new content, it is also the saviour for when the immersive experience passes it's expiration date. I really hope that Blizzard introduce flying a lot sooner in Legion than they did in WoD, because, honestly, by the time it did arrive, I was sick of the immersion (at least for the areas not Tanaan).