Saying that flying is a major factor for harming wpvp is like exclaiming about a paper cut on your thumb while you're bleeding to death from multiple gunshot wounds.
In other words: A gross over-exaggeration.
Yes, on occasion a flying mount will allow a person to escape from a PVP situation. But I'd argue it just as often allows you to get the jump on someone or reach a fight before it's over.
Either way, is that anywhere near as harmful as the thousands of people who never enter into WPVP at all because they're sitting in Shattrath waiting for a BG or Arena queue to pop?
You guys are so focused on this one tiny aspect that you're not seeing the bigger issue. To claim that flight is a "major" factor is completely false.