When the knights first landed on Cradle, the core exuded mass amounts of energy. This energy came in two formats: solid crystal energy and gaseous mist energy . This mist energy was so abundant that we could collect 100 units per day, and then throw the rest away like nobodies business. We had mist tanks and mist wells to hold it all. These were the LSD times. But one day we all woke up from our privledges and realized the mist disappeared; the core was no longer emitting such strong energy readings and energy now can only be obtained in it's solid state of matter.
So game changing dev notes aside, what could have happened to change the core's energy expenditure?
Could it perhaps be getting... weaker?
To further stretch a point, all elevators that needed power before have become charged indefinitely as to not require energy anymore.
So maybe not the sense that all this energy leaked was getting weaker, but rather more integrated.
The core wanted people to explore more.
And buy energy...
You know.
Instead of powering mecha knights and going to danger rooms for free.
Instead of bribing krogmo...
Instead of making gear at discount prices.
Because the core needs money... lots of money.
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TL;DR
What are your theories for the canonical disappearance of mist?
Honestly it feels kind of wrong to do this... like, "theories" implies that there is some explanation that was created on purpose.
Energy cannot really make sense; it transcends and breaks the plot in two by nature of the fact that it was created entirely to monetize the game. If we are to look at it, we must ask where it came from, and then what we're paying for it, and that implies that knights have real world human currency that they trade to "the strangers" for the energy they receive. While it would be interesting to have the plot encompass this, the game isn't set up to do so. Beyond that though, energy wasn't created for the plot, it was forced upon it as a means of making money for the game. There cannot be theories about how energy fits into the plot because it just doesn't.
The best canonical explanation would be that the half of the knights realized that their goals were meaningless and stopped attempting to go down in the clockworks, flying off in small personalized space ships to other realms with newer and more entertaining goals. The strangers (and Spiral HQ) panicked, and stopped taxing the elevators to make them come back, and then colluded with the gremlins + swarm to create rarities, so that they could all safely consume the rich diet of human money that the knights magically secreted with their plastic cards. When that started to fail a bit, they had one of the strangers, Ian, create a huge number of flimsy but good looking armors to sell (via "Spiral HQ") using gambling tactics. Now some of the knights are fed up with the stranger stranger's stranger decisions and are leaving once again, but since in the past the knights have been angered by the stranger's words, the strangers are too scared to talk to them. I hope I didn't forget anything in that?