Flying was a huge factor in the death of realm communities and real social interaction, but not the sole factor obviously. By the time of WOTLK there were NO realm communities left, let alone WOD and Legion.
The reason people were much more helpful and willing to go out of their way to assist people in vanilla was because the world was tough and unforgiving and you died a hell of a lot more. Not so in the later expansions, you could breeze through without much of an issue. And let's not forgot about the class homogenization and the removal of buffs so you can't even buff random people, not that they needed them.
Let's list a small number of what not having flying would encourage you to do when the world wasn't so easy.
-Buffing people
-Ressing people
-Grouping up for quests / difficult areas
-Mutual protection
-Giving advice
-World PVP
etc.
Most of these are no longer applicable to retail even if you remove flying.
-Can't buff people
-World is so easy most people don't die or if they do a graveyard is 30 sec away
-Almost no group quests or hard areas
-Quest tracker tells you exactly where to go
etc.
Due to how much the game has changed, the only thing delaying flying mounts does is encourage world PVP. So yeah, while flying played a huge role in the destruction of communities it was not the sole reason.
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What complete garbage.
People were MUCH more helpful before flying. When pulling 2 mobs meant death, and death was a 5 min run from the single graveyard in the zone (if you're lucky enough to have one in your zone), when buffs were incredibly helpful and powerful and you helped someone you saw struggling because you knew death was a real punishment.
Realms were on life support long before they implemented CRZ.
https://www.youtube.com/watch?v=l0ygAfTYKXk
I suggest you watch Preaches Top 5 WoW Regrets, skip to 11:40 for the part about flying.
I genuinely can't comprehend people who whinge about not being able to fly. I'm guessing these are the same people who are happy that WoW has slowly been changing into more of a singleplayer game with MMO elements year after year.