Is adding more worlds going to actually fix this problem?

[Zun]

Simply expanding the servers would help resolve the issues, FFXI had huge server worlds for quite a bit of people. If the worlds seem to crowded when we get in, then us as players should get on the forums and let them know so they can make more (with one-time free transfers for people who want)

FFXI used far less bandwidth. It was also designed to accommodate players using 56k dial-up connections (because those were still a thing back then). Much less data throughput per player = more players capable of existing at once.