This is a problem with most online games/MMOs, the last MMO i played was doing the same thing, FFXIV where once you "capped" on progression, there was little reason to log in outside dailies and raids (and once those were finished, you logged off until those reset)
It's not something they can easily fix, people get bored, content can only last so long, and due to how this game works. there really isn't much incentive to log in outside dailies, that is why they have these big rewards campaigns for just logging and doing UQs, because they likely know people won't stay long. I am also seeing the effects again as half my alliance stopped logging in since they got what they wanted or nothing was dropping so they lost interest.
JP version didn't escape this btw. they also went through drought periods when Limited quests were down so no one logged in outside of worthwhile EQs.
Since NGS is right around the corner, most of the focus is probably on that. so don't expect any major new content for PSO2. and since your gear won't matter by then, lots of people have realized this and also stopped.
In short, there is nothing they CAN do about it, take a break and play other games.