It is expected to happen in MMOs and Online games with weak single player incentive to play, for once the game-only social connections vanish, a player will be left with hardly any incentive to continue. Those without creative and/or immersive reasons to play beyond digital goods or numeric goals, will often burn out even faster.
Alliances and random active recruitment partying can only go so far, if the player cannot establish a strong connection with another to enjoy the gaming experience with. Those who create an alliance are also vulnerable to this of course. Should the leader and/or officers burn out for any reason, it'll crumble within on activity.
That's pretty much the main reason why I have yet to join an alliance. I want a leader who is truly passionate in playing the game in more ways than one. For now though, I'm enjoy what I do even if I'm playing single player fashion in PSO2. But should it all become boring for me, or my closest friend that I enjoy playing with as a duo, it'll get shelved for the time being.
Of course trying to make new connections is a goal, but that takes more luck and opportunity than most people realize. Treasure your experiences when they're good.