There are quite a few people who continuously pop up and claim that flying is the worst thing to ever happen to wow. While I don't think that qualifies as a "appeciable push", when combined with Blizzard's stance on the subject, it's definitely a concern.
To be fair, it is within a player's rights to cancel their sub if they feel strongly enough about a portion of the game. I also think that when people say they're cancelling because of No-Flying, they they're often referring to the larger issue of Blizzard's entire approach to the open world. It's not JUST no-flying, but the increased grind and tedium replacing a feeling of freedom and efficiency across the board.
There's also a side helping of feeling like Blizzard will not listen to anything other than a cancelled sub. This is only reinforced by the correlation between dropping subscriptions and the complete 180 on No-flying that happened at the end of WoD. This isn't really within the scope of the discussion, but I do think many players feel like cancelling is the only way to get through to Blizzard.
Well, largely in part to the fact that players had the entire span of WoW from TBC to MoP to get used to the idea that flying was part of the progression of their character through an expansion. Additionally it provides more options for content design. I've cited a few times that the formula of mixing an expansion's open world content with both flying and no-flying areas adds variety, and allows players seeking different things to be able to enjoy different types of content.
We have dungeons for small group content, raids for large group content, plenty of solo content in the open world. We have lots of different choices for playstyles via classes and specs and gear. We have optional stuff like fishing, crafting, archaeology, and pet battles. There's the choice between pvp or pve servers. Battlegrounds or arena.
And yet for some reason, in this virtual cornucopia of choices for how to play the game, Flying is somehow singled out as something that can not be a player decision?
And in some regards they would be justified. After all, if they've already completed something on a main, what's the purpose(other than personal enjoyment), of forcing them to repeat it on an alt? The game is rife with catch-up mechanics: Herilooms, nerfed difficulty in previous expansion content, paid level 100 characters, DHs that START at 100, etc, etc.
It's a fine balance to strike, but in the case if Flying I think Blizzard is going overboard for very little benefit. To turn your own question back on you: What benefit is there to restricting flying? It allows Blizzard to put the open world on rails, but has the downside of basically telling the players that they're not trusted to make their own decisions. It allows Blizzard to cram more content(and I use that word loosely) into a smaller area. But are smaller areas really better when talking about the larger, open world?
There are LOTS of benefits to Blizzard. But where are the benefits to the players?
I think if you take the time to talk to most Pro-flying players, they'll happily tell you that they're ok with flying being locked behind some form of achievement, preferably some kind of epic quest line that fits it into the lore. What they're not ok with is locking flight behind an arbitrary time gate that has nothing to do with player progression or effort, and has a list of requirements that have nothing to do with flying, and locks flying behind a point where it becomes worthless to have.