There are plenty games that are "fixed", but are still ridden with RMT. The game being fixed has nothing to do with it.
Yep, completely pointless to allow people with more revenue to pay the monthly for people with less.This is a pointless system for users - why do we need it when everything we want is obtainable through moderate play?
Oh wait...
Completely different system with completely different objectives. A cosmetic store simply serves to generate additional revenue (and it's an extremely annoying way to do so, as the developer needs to dedicate plenty development and design resources to keep the store full with items, instead of creating the same cosmetic items, but allowing everyone already paying a monthly fee to enjoy them). Chronoscrolls/PLEX combats RMT and helps retaining more accounts without the slightest expense from the developer.Using the "But Mom, it worked for EVE and her friend TERA down the street!" mentality - Why not do something a bit more interesting, more Final Fantasy? Why not implement a system like the digital store in Guild Wars 2 (minus the boosting items)? Maybe do it via the Gold Saucer. Spend Gil or real money (which has a controlled and moderated conversion rate) on tokens to play mini-games for character cosmetics or a new rug for your house. Now the developers have more money and I have a badass Mithra rug.
Retaining accounts isn't just a matter of additional revenue, but also a matter of keeping population stable and/or growing.
People in the game aren't just paying customers, they're also effectively a form of content for the other players to play with. A cosmetic store does nothing in this direction (other that annoying a lot of people).
Unfortunately the P2P market is going in that direction, but I won't sure suggest it, when there are better solutions that have additional perks on top of additional revenue.
You assume that RMT gil would automatically cost a lot less, which is not realistic, because the price of this kind of things adjusts between demand and offer.