There are hundreds of mmorpgs new, old, 2d or 3d that reset player rankings all the time and do not freeze the entire game to do so. Why does this game (and I'm assuming other Aeria games) freeze when the ranks are reset. "Because it works that way" is not a valid explanation, not only because that tells you absolutely nothing, but also because if that's the case just change how it works.
It shouldn't cause so much stress on the servers that people cannot perform any action in fact it should barely even have an effect on them, it's literally just inserting blank data where there was once filled data, how could that cause it to freeze? If you were putting complicated data in I could maybe see that, but you must have this system automated, and as I said it's just removing information or resetting it to it's default value. Would this forum freeze every time you deleted a thread? I doubt it, so why should the game be any different?
It shouldn't be the case of no one complained so we haven't done anything about it, if there's an issue and you can fix it, you should. If more people actually knew about this game something like this might turn potential customers away. I'm not quitting the game over this or anything, I'm just saying it's silly for the whole game to freeze, and something should be done so that this doesn't happen.