It's not just people who purchased everything. Its also people who purchased everything they want. It's also potentially worthless for several months out of the year for anyone who might have something they want since the shop items rotate in and out. Should you be required to hold your aya for a year plus? Should you be forced to by junk you didn't want just to make use of it? Plat is useful for everyone who has it since you can use it to buy new things, old things, trade it to players, use it to speed up projects, etc... Aya is only useful for old items and if the stuff you don't want isn't available you gotta wait months for it to rotate back in meanwhile if I want an old prime I can trade plat to a player for the parts, or if I want a decade old skin I can just go to the shop and buy it with plat.