I feel like the game has a number of creeping issues and it probably makes sense for the population to start to decline because of it.

Job design is always a major issue with every expansion, and anyone unsatisfied has to just wait until the next expansion.

The quality of the MSQ has gone down.

The graphics update has messed with character faces and from the updates we've had it doesn't seem like they'll be able to remedy it.

The content pipeline hasn't seen many positive changes, new things like variant dungeons and chaotic seem like they've missed the mark.

Basic stuff like viera/hroth hats have been on the backburner for years, I don't even play them but it's just plain embarrassing at this point.

For an MMO, if you're not growing your playerbase, then it's shrinking. It's very normal for people to be coming and going, the population is never going to hold steady. But the excitement and interest of current players is part of what draws in new players. If everyone playing the game is just talking about all the problems then fewer new players are going to be drawn in, and the overall population will decline.