XP Bonus Synergy: The system is there for a reason, making it easier and faster to get new Heroes to 60 the more Heroes you have. It offers incentive to collect more Heroes and leveling Heroes you think you don't like and in the end maybe be surprised and find a new favorite. Happend to me on several occasions. Leaving the XP rate as is it becomes somewhat obsolete, you level fast enough anyways regardless of the amount of Heros at lvl 60.
Prestige: Prestige was meant to being able to show other players that you put some extra effort into a Hero, leveling to 60 more than one time. It takes work and dedication putting in the extra hours and getting rewarded with a colourful name and a costume. Leaving the XP rate as is it becomes somewhat obsolete because it virtually takes you only a couple of minutes to to prestige one tier when done right. If everyone can do it fast and easy i miss the point of it being prestigeous.
Costumes from Prestige: See above... you get a free default costume for a couple minutes of "work" so you can "blend" free random costumes to your hearts content with not much effort instead of having to choose: "Do i grind alot to get the costumes free or do i give GAZ some money because it's easier than grinding". Leaving the XP rate as is i believe costume sales will go down and GAZ will be losing money in the long run, hurting the game.
Omega XP: Omega Points are rewards for playing a long time, working towards it steadily and improving your character via time invested in the game. Not only is the XP to get a Hero to 60 increased beyond reasonable levels but also the amount of Omega points you get due to the high quest reward XP and therefor getting Omega Points is even faster now than when the Omega Event is running (which is normaly intended to give the fastest Omega XP/Point gain). Leaving the XP rate as is the Omega Event becomes somewhat obsolete and in the long run the worth of the Omega system aswell because everyone can get high amounts of Omega points really fast.
Buffs and their significance: If you already level that fast .. why bother buying XP Buffs when there already is a XP Bonus from Events, Hero Synergy and Bonus XP Synergy? Leaving the XP rate as is XP Bonus from Events becomes somewhat obsolete because it is no longer needed. You can level very fast all the time without having to wait for a event with high Bonus XP. You don't need to buy XP Buffs anymore and sales will go down.
Hero Level and leveling: Some may say the Characters level in general in ARPG's is a thing from the past and there is no need for it anymore. I disagree. The continuous gain of Levels and step by step gain in strength of a charakter makes it in my opinion smoother to get a feel for a Charakter. Getting points to spend in the various character defining skills along the way of getting to max level is less overwhelming than having to figure out everything too fast or from the start. A player can fiddle with and redistribute points everytime he reaches another level and gets more points and skills available to put them in. By the point you reach max level you already know how the Character works, have figured out your own style and what works with your build, definded by the amount of skillpoints you distributed to certein skills by a certain number. Leaving the XP rate as is a learning curve is basically non existant and there will be no need for skillpoints and the next step would be to implement a uninspired, brainless system like in Diablo 3 which will hurt the depth of character customization.
Content: Leveling, getting Omega XP and Prestige together are content. Wanting to have a Hero at maxlevel 60, to have a Red or even Yellow name and alot, if not max Omegas... is content. It's stuff that makes you play the game. It's stuff you can do in the game. Leaving the XP rate as is you will run out of things to do much faster and therefor you will get bored faster. Noone wants to be bored.
All this is my point of view, my opinion and if you disagree feel free to let me know your take on the matter.