If Nexon doesn't intended to return the actual events these items come from they could easily add them in as generic event rewards (Like Fairy Heart) or as permanent additions to other currency shops. I do like the idea of adding them to Monster Park as eventually those coins become extremely worthless and just start overflowing in your inventory.
I think Nexon is tired of their staff being demoralized when they "fix" GMS content only for it to break in a few patches if not the very next patch.
These constant bug fixes for a small percentage of your profit just isn't worth it and it's easier to just face whatever backlash that comes with it.
GMS was bleeding Nexon money for years, now that they're finally earning money they probably want to streamline the development process for it and refine this version of the game.
Basically we got to keep our content when we were bleeding money because Nexon didn't care. Now that Nexon cares they're realizing how messed up GMS actually is.
The other stuff like Dimensional Mirror, Silent Crusade, and others are removed simply because they're useless.
All this system really does it punish people who play the game Nexon designed (multiple characters heavily encouraged) and don't have the time to spread out when they do each character.
I'd prefer to see the system changed to be completely free like legion.
