This would be nice whether population declines more or not...I have people on other servers who'd love to come to mine /join ls.
As to further population decline: Honestly, the people who are still playing will be the ones still paying. Add maybe 20% more SUB fees for those sitting on the sidelines in the lulls between patches. You know, the people who all come back right after a patch, finish all the content they are interested in, and then leave and go do something else until more content is added. They still like the game, they are just choosing to do something else with their time during patch lulls. (this 20% may or may not pay the Jan/Feb sub fees but will be back Feb/March with the 1.21 release.) After 1.21 release, we'll probably see another population increase as that will mark the conclusion of the battle system and class adjustments. (maybe 10% more will come back). After that, we'll probably have a steady loyal population until 2.0 comes out.
My point: I really do feel the population is about as low as it's going to get. There are a lot of people sitting on the sidelines just waiting for patch 1.21 and beyond. Once we get those patches, they'll be back, many to stay.