Jagex said every single treasure trail is now going to yield a treasure trail only reward. Your doubling the amount of rewards, but way way way more than doubling the chances of getting a reward. The outcome of the items will probably be like the staff of light, hexcrest, and focus sight. People hoarded like they do every new item, people payed ridiculous prices for about 2 weeks, then once people figured out it was actually a lot easier than expected to obtain the item yourself, they crashed to below where the item started in price.
A similar sequence of events will happen with new treasure trail rewards, only it will be much much more drastic. A treasure trail only reward every single time? After the update, I could probably get a dragon mask every day or so if I really wanted, but currently I could not get any single piece of 3rd age in a month of no-lifing hellhounds and treasure trails.