Firstly why would you pay a sub for WoW. It is immensely easy to make Gold in WoW and you can buy game tokens.

Secondly SE couldn't turn FF14 into an open world game if they wanted to. It would require such a huge overhaul of the games core engine that it just wouldn't be feasible. Its like trying to turn a car into a speed boat. Maybe you could do it but it would probably wouldn't work too well and in the end it would just be cheaper to pay for a speedboat.

Thirdly they are introducing flying into legion within a few patchs. They said mid expansion so that probably means next teir so 7.2 or 7.3 at the latest. Frankly movement around Legion will be quicker and easier than it is in FF14 relatively soon.

Fourth Blizzard has a habit of being far less consistent and more prone to complete overhauls. This applies to game design and story. The artefact you would be grinding your ass off in Legion to make won't survive into the next expac and currently they are saying they have no plans to create a new one since 'its a legion thing'.

Finally, both games have a lot of great things about them and both are worth playing. However while they have a lot of similarities they have a lot of differences too and weither you favour one of the other is purely down to personal taste which varies a great deal even within a single games playerbase. Case in point, I LOVE FF14's crafting and gathering system and it is superior to WoWs in a way that frankly I doubt WoW will ever manage to match.