The core problem is that killing standard mobs isn't compelling, it's why players so desperately take advantage of these various quirks. It's simply a means to gain exp to get to the more interesting things in the game. Massive stacks of mobs aren't a threat because of the way the maps are designed, so having such a thing is just a blessing and a way to get through that grindy non-content faster. If anything, they should permanently set the mob count and spawn rates high, or do the less risky thing and bump up the rewards so less time needs to be spent there (they've already done this a few times now and seem to be prepping to do it again). There are other games and MMOs out there that feature similar content that is made compelling by risk that rewards skill. The classes here are so tailored that blocks and dodges aren't something you can rely on in mob design (Flash Jumping is usually to large scale to work with anything but bosses), and again a number of the classes can also kill mobs from safe platforms.

Also, the game is completely designed around forcing players to make class after class, while also locking everything to punish you when you invest in a single character (especially end-game). NX transfer limits, pets, trade-locking equips, etc. But then link skills and limitations during events advantage players that grind through the same non-content over and over again. When it's you're 16th time grinding on the same map, you're probably pretty likely to formulate something to make it less of a pain. It's great to get people to experiment and try all of the classes, but they're extremely unlikely to really care for more than one in the long run because of just how much work a single character needs, and how much all of that effort/money does nothing for any other character but the one you invested it into.

In the current state of the game, breaking Kishin makes the game slower and thus worse (also annoying all of those who have invested trusting that useful character ability would be there). But in all honesty the game needs substantial reworking before small changes like this are actually relevant. Its like complaining that the ball is a bit dirty, when the pitch is flooded and you're knee deep in water.