i think the idea here is more about using NPCs to regulate inflation and deflation than using a cpi index to match all npc prices to market prices.

And this is not an argument for a total free economy because if it was you would eliminate NPCs out of the picture

The only problem i see with this idea is that it would lead to constant inflation overtime. And at some point RMT would take advantage of the system to totally mess up the economy.

For example if I was an RMT i would pick one ward (prob food for fish) and make sure i have as many bots as possible in it. I would then spend a day selling stuff to myself at extremely inflated prices. This would drive up npc prices and payouts by a massive % compared to other pieces of gear. I would then unload thousands of items i have stored up on RMT characters to the npc, make hundreds of mil of gil and hold on to it. The wards would drop down to former prices (due to supply and demand without a major increase in overall currency) and I would have a lot of gil when other people don't. then sell, get rich, repeat.

so while its an interesting idea, i think it would take a lot of work to regulate it. Thats not saying it can't be done. But since I'm pretty certain the FFXIV team lacks anyone competent in economics, it would probably fail.