It's not a matter of balance. How 'strong' the first event unit rarely ever matters and in the case the unit or event is unbalanced, then developers can simply adjust the units parameters or make changes for future events, which as you can see in the post above, they do by reducing the amount needed for future content, which can only imply the struggles of trying to complete these kind of events will most likely be everlasting within the game if they had to reduce it and not increase it.
Regardless, this will only leave a bad impression on the game for some players as completionism and collection are a big part of these games. Only being able to partially complete a event girl/weapon/etc no matter how much time and effort they put in will most likely deter them away but hey, who cares about other people right?