My guess here is that more gold is being generated than plat, so inflation in plat price becomes reality.
Sheepdog I feel like this is because despite it being abundant, it is the only way to obtain certain items and extras typically aren't in circulation short of someone quitting. If you could buy these platinum rewards with gold at 10x or even 100x the cost of platinum coins without having to obtain platinum, the price of platinum would plummet. Its price hold steady and increases because people aren't buying extra blessed runebooks or extra ethys just to sell for gold. Because why would you when you can horde your platinum and maintain or increase it's value over buying items you don't need?
While I like the Robe of Invisibility idea, it's essentially giving you free skill points. It takes at least 60 or better magery or hiding for invisibility to work in the same situations where a robe would make you invisible. Sadly, it wouldn't be in balance with this era. Or for it to be, it'd be such a ridiculous option like the cooldown being a minute or the charges being less than 5 or something wacky.
Thanks @Sheepdog. And let's say you start farming the plat from slimes or the likes that give you 1/200 chance of a drop. That's 50,000 slimes you've gotta kill for a blessed runebook. Why slimes? One of the highest drop rates. So now you've gotta recall out every plat because if slime farming for plat becomes a thing, PK central. Thusly buying it is easier.
I would like to see weapon and metal armour dyes added. Different colored mounts instead of just ethys.