I always thought Big Bang was the downfall of MapleStory but looking back I do realize a few things. Mechanics wise Big Bang was great. The EXP grind was unfavorable for many players and was usually one of the turn-offs for MapleStory and while I don't agree with accuracy changes it did sort of get it right. Then there's potential, which many believe was the downfall of MapleStory. While I agree it did turn the game into a cash cow I don't believe this is what actually killed MapleStory.
Have you ever noticed how popular MapleStory was? Over 5 years ago MapleStory's community was huge. There were many forums and websites related to MapleStory that were active, even good old MapleTip had a sizable userbase. This popularity can really be seen on Youtube videos. Look at any Maplestory video from 2006-2008 and I'm sure that it would have at least 1,000 to 10,000+ views.
Then there's dray86's videos which have upwards to 1,000,000+ views. I mean holy crap people actually cared about MapleStory around this time. When people reminisce about MapleStory they usually mention their friends they met. Around 2006 - 2008 people actually did interact in-game and with the community. Even me, who has troubles making friends in MapleStory nowadays had a sizable group of friends back then.
In this period of time people just don't want to socially interact with each other. They simply want to level their character, or their link mule or increase their damage through potential. This is why people believe Big Bang killed MapleStory. However I don't think that it's to blame.
Big Bang never forced these ideals onto you. Nowhere in the huge mechanics overhaul did it require you to fund your character with $600 worth of equipment to have fun. The main problem with MapleStory right now doesn't stem from Big Bang at all, the game is hella better mechanics wise (engine wise I still don't get why I have 30 fps average) and we have Big Bang to thank for that. No, the problem as I see it is the community. I really don't even see a sizeable community anymore. The number of MapleStory fansites has severely diminished. Many sites have either died out or are on their death beds (mapletip, sleepywood, etc.). The remaining sites see nowhere near the amount of activity as they once did.
So did Big Bang actually kill Maplestory? No, we all just simply grew up. MapleStory just isn't as popular as it once was and the player base is slowly diminishing. The remaining player base is older and have experienced either fully experienced MapleStory or just doesn't enjoy it anymore and just focuses on raising their damage. When you were young it was easier for you to make friends, and I believe the same can be said for MapleStory. In it's youth people were friendly and interacted with each other but as we, and the game, grow older we find it harder and harder to make friends.
Tl;dr: Maplestory's problems never stemmed from big bang, it's simply the remaining player base has grown too old and MapleStory's popularity is diminishing.