Considering you can’t sell platinum for money, you could add complexity by converting it to another currency when exchanging hands. No value lost, exact same ratio. You buy platinum, you spend it on the store or it decays when you give it to another player. Platinum carries real world value, decayed doesn’t. Would that work? The only reason for doing that would be to obfuscate the fact platinum has real world value. The players being constantly aware of the fact might mess with the economy.

Honestly, their monetization is really something I could never criticize.