Hopefully half the player base will leave, as it will probably make them address the issues.
That being said WoW had a number of issues the led to it's decline over time. First and foremost is most people played WoW because it was a follow up of Warcraft 3, so WoTLK was its highest peak and it fell after that as the Lich King plot was completed and that is what most players likely played for. Cataclysm was in a way like HW where the content was way overtuned and very difficult so players began dropping from that. MoP was the great pruning where classes began feeling very similar much like SB and ShB, players began to drop off from that. WoD on a big issue over all was lack of content. The game spiked with Legion, but quickly fell off due to Blizzards artifact power grind they needlessly kept in expansions afterwards as well. When you look at the two games, they follow a similar flow, and really FFXIV has not learned from WoWs mistakes at all. They made the game based off of WoW and it is going to eventually fall apart for the same reasons as WoW.
We are at EW where I will guarantee a large number of players will likely not be returning as it's finished the story they started in 1.0 or 2.0, and it now picking up a new story which some people may have no interest in at all. We have also slowly been seeing the same problems that WoW has had with the same solutions WoW offered. We are seeing content cuts with dungeons but nothing in return for these cuts, we are seeing class pruning and homogenization. The only thing so far that has been different is this games consistent patch cycle, but for many people that have played this for over a decade that is becoming stale and redundant. I am waiting for when we suddenly get a daily grind fests as a last ditch effort to keep people subscribed.
They actually are not different in nature. They are to make classes more player friendly, and to homogenize the classes. This is the exact reason WoW did this as well, which they eventually fixed for tanks and healers but DPS for the most part is pretty terrible to play in WoW. Currently WoW has more fun gameplay if you want to tank or heal, FFXIV has more fun game play if you want to DPS. With the route FFXIV is going with classes though, WoW will just end up having better game play.