I've seen this argument brought up multiple times. That Square Enix can't just buy new servers because "the population will go down after expansion launch".
This is a "self fulfilling prophecy" way of thinking.
If SE buys new servers the population might go down. Or not. Cause the people that played the game could recommend it to their friends and bring over more people. Any investment is a gamble after all. You can win or you can lose.
If SE doesn't buy new servers, the population WILL definitely go down. Because of people who were not able to play leaving for other game where they treat their playerbase better. No investment leads to stagnation and decay.