When CE is sold on the current crown-CE market, only 2% of the crowns are destroyed through fees to Three Rings. The remaining crowns are transferred to another player. Typically, people who buy crowns with CE are doing so because they don't have time to play levels for hours and hours and hours. Wealthy players trade a bit of money for their time.

If Three Rings created a CE Tank, all crowns would be destroyed in any crown-CE transaction. This would upset the current economy significantly. There would be a massive drop in the number of crowns in existence, making things like guild hall purchases and punch rolls much more difficult (activities that wealthy players often use). In addition, it is likely that auction house prices would drop significantly.

EDIT: I just realized you may be suggesting that Three Rings only fix the CE price, and that the CE would still be sold by other players. That would be interesting, but I think a free market is better. People are motivated by profit.

All you have to do is take Macroeconomics to know that imposing a price ceiling above the equilibrium price (currently around 6.5k-7k Cr) will effectively do nothing since the price wouldn't normally go above equilibrium anyway unless some variable changes. However, if their price ceiling was set below equilibrium, it would cause a shortage since more people will demand it and fewer people will want to supply it.