There will always be a market. The price just goes low enough to be irresistible.
Irresistible?
After a quick search the current price seems to be around $0.90/1 million gil.
Most relevant gear like Su5 weapons or necks will quickly cost 40-60m gil, not even counting heroism crystals.
For 60m gil on asura they want $55.
Business 101. You don't pursue a maybe at the expense of an absolutely.
Unless there is potential for more money to be made.
The only real threat to RMT are the game companies themselves so SE is in complete control of the situation and I don't know of a single game company that accidentally scared away RMT with a unmindful decision.