To understand why this is necessary, you need to understand the history.
Before Shadowbringers, Balmung had been congested for years. It was so congested that creating a new character on it was technically possible, but only for about 1 hour a week at the lowest activity moment of that week and you had to plan it in advance. People got around this by doing paid transfers, and I made a linkshell there which filled to 128 new transferees in just 2 days. At that moment I knew the bubble was going to burst eventually and something had to give.
So SE invented "Congested" status to make paid transfers impossible as well. Combined with splitting Balmung from Aether onto its own data center (Crystal) with lots of low-pop worlds and adding World Visit, they effectively pushed people off Balmung. They either transferred to Aether to raid or they moved to other Crystal worlds, so that Balmung's reputation for RP could spread evenly across the other worlds on Crystal.
I know that you want to believe that servers can grow endlessly but that only happens because of cloud servers where you are all technically connecting to different servers that appear to link up. That's how Discord works for example and it's why bots go down on some shards but not others.
As for the matter of population, I can say that Aether and Crystal's active population often reaches 200,000, which is way higher than after the DC split and Crystal's creation. Crystal's population often surpasses Aether's quite regularly now. According to the last update by Lucky Bancho, the population of Crystal was as low as 138k, Aether at 130k, Primal at 120k and Dynamis at 50k.
Obviously this likely jumped due to the patch, so Aether and Crystal would be higher since then, and it therefore makes sense why they would want to push people elsewhere. And obviously they want to push people to Dynamis generally.