Xp rates need to stay at the normal xp rates. Otherwise, you'd have a bunch of people that have maxed mains, or maxed pures.. boosting the XP rates will destory the attention getter for the game of Runescape. Most people who want to invest their time and money into the game want to get a bigger/better return than others that haven't invested as much time/money into the game. If you were to boost XP rates, then everyone would be the same and no one would have an advantage in stats because everyone would want to get to maxed level and with the boost in XP rates everyone would now be able to get to maxed.
Also, the game of Runescape is an 'unbeatable' game as I like to call it. That's why it has so many players that keep returning to the game year after year. I alone have been play Runescape for over 6 years now and find it hard to not to return to such a game. If you make the game to easy then people will lose interest much faster.
At first people will love the XP rate boost, but then after they've beaten the game, and maxed out. Then what? Most of you will say go PK and own the wildy right? No, Wrong. The wildy will be filled with players that thought the same way you did and now are maxed just as you. You will then end up losing because then it will turn into a game of 'I have more friend's than you do'.. a fucking pissing contest.
What I am trying to say is that boosting the XP rates with be fun at first, but then after you realize about a 100K of other people thought the same thing as you and maxed their account you will losing interest.
Normal XP rates will suck at first, but after a couple of months of training your perfect account you will be one of a kind, and you will be able to own.
Also, another thing to think about, when you play Runescape what are you doing? Training your account. Once you max your account then you stop training your account because you can no longer advance in level. THEREFORE, you stop playing the game of Runescape as much. And that is something Jagex doesn't want.. Jagex will always keep XP rates normal. Actually I would be surprised if they cut them in half..