While I agree completely with what you say, for me flying is also a form of content. Most of the screenshots in my gamefolder are from sunsets/sunrises and amazing views from above.
There were expansions with tons to do, MoP was in that regard, for me, an amazing expansion, I did rarefarming routes (maaany rarefarming routes), herbfarming routes, soil farming for tillers, the dinosaur riding guys farming routes (don't remember their name), and ofc flying to the questhubs with some guildies.
Plenty of content. And flying just made it more efficient.
The difference with WoD and Legion is so big. I have played for almost 10 years, and never have I been bored, but in both WoD and Legion I have been bored, altough for different reasons.
WoD just didn't have much to do, flying or no flying would not have made a difference. I quit over the 'no flying', but I probably would have quit some time later anyway, due to it being such a boring expansion.
For Legion however, I felt like there was plenty to do, even if it was just class or artifact stuff, altough it was getting repetitive, especially when playing alts as well. Until one thing bothered me more than stuff being repetitive, it was doing repetitive stuff on annoying terrain, a mountain between me and my objective, or a cliff, or many mobs, or stupid roots where any horse or car could drive over, but not in WoW horses can't jump over angle-high roots. I unsubbed again over no flight a couple months ago. I was excited for them to have a 'flight-plan' for Legion, I did the first part, hoping the next patch would unlock flying, and since it didn't I quit.
Probably more because I hope flying will make the game excited again, or will 'add content', or maybe something else. But the conclusion for me is the same, if the game had plenty of fun things to do, I shouldn't be bothered with riding there in stead of flying there.