Yoshida mentioned in one of the live letters that they are working on social cross-world instances for role-playing purposes.

Locked worlds are entirely population control to prevent server instability. You can say no evidence but they aren't required make all of their data public so it's a dead end. Server equipment does have its physical limits of course and S-E have decided where to draw the line on max simultaneous users so that the gameplay experience is not severely impacted by lag and disconnects.

Balmung has been over the edge in the past where user experience was heavily impacted, like crowds of people in one place losing connection to the server all at once. At the same time, datacenters with less congested worlds were not experiencing problems. They had to encourage people to transfer away with big rewards and then lock it down to prevent ever getting back to that point. Bottom line they will not open worlds unless population decreases.