I don't think it's a matter of locked servers. The elephant in the room is that SE doesn't do a good job of getting people to stay playing.

The content model we have encourages raidlogging, and stormblood made it worse by being more or less an extension of Heavensward, not it's own expansion. By this I mean that we do more or less exactly the same things we did in HW, with no real variation or unique aspect to the game. While people leveled their mains, the game was busy. Now that people have done so, there simply isn't enough fresh content to keep people playing.

I think SE knows this too, which is why they wound up repeating a couple of past events. Even then, the current content model is tired, and on locked servers, there aren't enough newbies coming in to replace the raidloggers/patchquitters. But if the current population were still playing, there's be much less of an issue.