In my opinion, the sudden departure of so many players (530 on Quetz at the moment) is due largely in part to XIV's now successful re-launch, I can say this with some certainty, as my XIV linkshell is made up largely of XI players. I do notice on weekends that the XI numbers still largely remain the same as three weeks ago (1.5k+ here). So in my very informal survey, people haven't outright left XI just yet, they're just not playing as much.

I think it may be premature to call for another server merge since XIV is brand new (again) and I'm sure many people who play both games are enjoying all of the new content in XIV, while they're likely caught up to what XI has to offer. As people reach end game in XIV I wouldn't be surprised if those who play both begin to spend more time in Vana'diel again, especially with each major update.

I of course don't have access to official stats or anything, but this sudden drop off doesn't seem unprecedented, I remember a similar pattern when other, non SE titles came out such as Diabolos 3, Skyrim, etc that have a big cross over appeal. Another major XI update, and people came back, Adoulin picked up steam and the game seemed more alive than ever in recent years.

For financial reasons, if I'm continually wanting to play XI and XIV, I eventually may have to decide between them, and would love to see some sort of packaged discount to do so as I believe the free to play model wouldn't work with XI's structure.