I know this is probably still basically a ridiculous question, and in most contexts I would look at it as laughable, but given the particular circumstances surrounding PAD, how important is balance? Obviously it's preferable to have a correctly balanced game than an incorrectly balanced one, but given that there is no PVP element in PAD and the whalings of some players don't really directly effect anyone else at all, does balance matter that much? If some game elements are just better than others it doesn't really cross over from the haves to the have-nots in an easy to see way, at least from what I can tell.
The power creep in dungeons seems like the biggest factor here to me, and for the most part it doesn't seem like high-end content is balanced around the absolute most powerful things you can do in the game, but more around trends and the rough power level of available monsters overall. What do you think? Is that off-base? Does a sudden leap in power in one corner of the endgame (combo leaders) have effects that extend to anyone beyond people using combo leaders? Does it even go that far, or does it only effect people using Bastet? It's not like existing combo leaders just got nerfed, even if they are now significantly weaker than another existing option.
Do we expect that this change will result in dungeons that are difficult enough that the others stop being viable, and if so, would those dungeons not have been as difficult without the Bastet change? I don't have a great grasp on absolute endgame content or a nuanced understanding of balance changes across the history of the game, so this is hard for me to piece together, but as games go individual specific balance issues in PAD kind of seem like they... Don't matter that much, honestly.
TBH, it's not really about balance issues, rather it's about money. In fact, the concept of balance doesn't really apply here.
IMO it's about the challenge. Think about this, if people always have an insanely strong card, wouldn't it already be boring for them? You should've noticed, if we started playing around a year ago, and with our current PaD status(rank, cards, etc.), we would already be considered in the late-game. Though we didn't played back then, and the late game back then, is actually the mid-game now. Some will say it's still the late-game, and say the latter dungeons are super late-game.
The point is, if they already surpassed the so-called "late game" long ago, it wouldn't be surprising some players will go on hiatus; worst case scenario: quit(hard to think that way though). So they go make an insanely good card, and make harder dungeons. It's up to the other players who doesn't have the viable cards to do the dungeon, whether they want to do it or not. It's not like it affected them directly, as no actual change was brought to the game overall, rather than just being another "added content", everything that was before it, remains the same.
Like Sacchi said, it's mostly about money. The real target is the IAP players. Non-IAPs supports the community, while the IAPs are the ones responsible for bringing food to GungHo's table.
Edit: Misspelled game. LOL