Nexon implementing this, is almost like star-forcing Kanna's Treasure ring past 17 stars. There's no turning back if they decide to make moving cash items between class branches unrestricted.
Let me explain.
I do agree with you that being able to transfer cash items unrestricted would be nice. I'm more reluctant to purchase more outfits since cash items can't easily be transferred over to other characters. Due to this, I stick to only one outfit each for the two characters I play often and avoid spending anymore than I need to. However GMS needs to earn some sort of revenue somehow, and with Reboot being the the most populated world when compared to the other worlds, most of GMS revenue likely comes from cosmetics and pets. Right now, Nexon doesn't really have a way to know for sure if adding this feature would result in players spending more money, or if this would actually result in decreased spending.
Perhaps players would be willing to spend more money if they know that they could transfer cash items between class branches without restriction. However, once some players have acquired enough cash equips that they are satisfied with the amount of outfit combinations they can make, and spread outfits between all the characters they wish to play, they may stop spending money in the game all together. If Nexon notices that it results in less spending later in the future, they can't exactly revert cash items back to being locked onto one character again.
Whenever something beneficial is add to the community, it extremely difficult to remove it without causing backlash from the players. A prime example of this is Kinshin. Due to the benefits of being able to increase spawn rate and limit, even making any negative changes to the skill's ability sparks outrage throughout the community. Nexon is pushing for the removal of the Kinshin from GMS, but are also aware of how the GMS community would react if it is completely removed. If the ability to transfer cash items and chairs without restriction turns out to harm their revenue stream significantly later in the future, Nexon simply won't be able to revert back to locking most cosmetic items to a single class branch without starting another dumpster fire. Having a large decrease in revenue stream may also result in Nexon making decisions about whether it is worth continuing to operate GMS.
From the linked PDF, you can see that Online PC games hosted by Nexon North America make up a small portion of revenue when compared to the likes of Nexon Korea. If the total revenue drops to the point where Nexon deems that GMS is no longer worth investing resources into, they'll pull the plug on GMS.
I don't know about you, but I prefer to wait patiently for the next cash item transfer rather then risk the chance of having GMS getting shutdown earlier. That said, perhaps making this change could actually boost the revenue stream, but no one knows for sure. Unless there are some similar case studies (where a free MMO that has no P2W, no longer locks cash cosmetic items to one character) to base reliable predictions off of, it might be safer to leave things as is for now.
The reason why Nexon probably locks event cosmetics and chairs to one character is so that they don't have to deal with "The event items I earned are movable within my account, why are my cash items not? Why are the cosmetic items that cost money restricted to a single character, while the free ones are not?".