People have been migrating from WoW since the beginning of FF14. Even in 2015, people I would meet mentioned they used to play WoW. I've talked to veteran players for years in the Novice Networks of many servers and a large amount of them would tell me that they either still raidlog WoW or used to play it and quit. The migration didn't suddenly begin a year ago. My assumption was always that 50% of people used to play WoW or are playing both.

There isn't any evidence that they bring anything out of the ordinary to the game that people from other games don't. There are a lot of people who began playing FF14 after playing League of Legends or Overwatch and I can't imagine it being any easier for them to adapt to the game. It's also worth mentioning that just because someone used to play WoW doesn't mean that they didn't also used to play 10 other MMOs.

A lot of people from WoW quickly discover the culture is different in the first few dungeons and that they can't apply the same mentality to it, so they just try to discover the game instead.

Most of the criticism comes from veteran players who have built up frustrations over time and are coming to the forums to release it all, but there are plenty of people who haven't had so many things bother them for this long, so you can't say all veteran players are necessarily like this.

The forums were full of criticism even before Shadowbringers but I don't recall doomsaying. The doomsaying is a recent thing and a bizarre exaggeration for a game that has added almost 1 million to its average monthly subscriptions in the last year, its biggest growth from any expansion release.