Personally, I think this would BOOM Jagex's profit.
Take a look at another game that implements this idea:
Conquer Online
People are constantly buying "dragonballs" in that game for $13 or whatever (I haven't played it in so long) and prices haven't risen/fallen drastically for years.
Towards the people who believe that the value of gold would drop drastically: you're wrong. There is only ONE tradeable value of gold in RuneScape and that is gold. There has ALWAYS been an infinite stock of GP in RuneScape. Every drop you get has a gold value, yet no one seems to realize this. The value of gold would not drop, as it can not drop. The 2146M limit will stop items from increasing drastically as well.
If Jagex were to sell gold to people, people would buy. Prices MAY start to rise for the first few weeks afterwards, but they would drop back down to a suitable price range. The RuneScape economy should NOT be based on a real economy, as real life economies are based on other items, such as gold standards and what not.
The Runescape economy is not primarily based on RSGP.
Other games can successfully implement this idea as it was thought about long before. The market is not a witches cauldron and it doesn't randomly spit out prices. It is a very complex algorithm with lots of things interweaving with each other.
The Runescape economy is primarily rooted in the price of rares. Rares are the biggest cash sink in the game, and they are forever rising due to their discontinued status. The fact is RSGP isn't generated in Runescape to any significant scale, the only way it is made is via Alching, stealing, and picking up/selling to shops. This doesn't create much RSGP.
If Jagex where to sell RSGP directly, the amount of RSGP they would put into the game would be cataclysmic to the market, as what will happen will be the EXACT same as the current market. The value of GP will diminish, and the value of goods such as Ore, Bars, Logs, RARES will rise exponentially.
If you know very very very basic economics you would be able to work out that if Jagex did sell RSGP the economy would spiral out of control.
Also there is not an unlimited amount of RSGP, RSGP itself is a commodity. There is an innumerable amount of it yes, but it has a limit. When you pick up say an Abby Whip, you are not creating 1.1million RSGP, you have just created an item WORTH 1.1million RSGP.