I personally love flying now that I only play WoW as an action RPG (raid and m+), but the opt out solution doesnt really work here for the people who hate flying because it breaks immersion.
The thing is that in vanilla era, the world was lively and immersive because you could see people everywhere. If 98 % of the population is flying around, you don't see those people anymore. Flying also kills the world pvp excitement.
However, I think flying or no flying, immersion would be in the past anyway. Removing flying would not bring the lively world back, as we have moved from 6month+ adventure into level to max in a few days and join instanced content.
In Vanilla you sometimes didn't visit a major city for several days, now you basically have no reason to leave them and if you do, you probably want to use the fastest transport possible to get back faster.