While I do find some merit in many of your points this would still end up pay to win. There are plenty of people that buy runs and giving them a legitimate way of essentially buying the gil would only make that happen even more.

And while the gil supply might not increase, the concentration of where it is could still cause inflation. The people buying gil could easily push up the prices on a select few items, and when they run out of gil they just sell more game time to the people they bought the stuff of of and the gil would just keep changing hands between very few people essentially locking out most of the rest of the playerbase from those markets.

This might work in EVE where the market is a big big part of the game, but I don't think it would work in FFXIV. Even WoW with it's bigger playerbase might be able to diminish the effects but I could see this very easily wrecking some of the smaller servers here.