First, I don't have any issues with the amount of XP earned, more with how it is earned.
My premise here is about having fun in game.
I think it would be safe to say, most players will find their epic destiny that they love to have for their character and not want to leave it. These additional abilities gained is fun for them.
What is not fun is having to leave that ED to go into another one to gain something else. It is one thing if it is the next ED over, but many times it is not. This means the player isn't having fun because they knew what they could do but currently can not. Power gamers will just deal, but average players this is a strong nitpick that will aid them in the desire to stop playing.
Players also like growth, and more than likely want those twists, but I bet a bit under than half just aren't going to bother. This means they will cap their ED. No biggy, BUT then they will always see that XP going to waste. Unlike previous caps, there is a very obvious area for that XP to go. This waste will irritate people and by association, add to the straw going on the camels back.
What I would suggest is that once your ED is capped, if you want to start to earn more ED AP, you be allowed to "Make Secondary".
A Secondary will only earn AP, but no spending it. You would have to make it primary to spend and set it up if you want to twist in fate, but this way you could move around. I also say "no spending" that way the coding doesn't get super tricky.
Also, I would say any "Secondary" only gets half XP, not the standard full amount you would normally get.
This way the loss of XP won't be felt nearly as bad when just having fun, and players may have their continued fun in the ED that gives them the most enjoyment.
Now.. catches that I'd be willing to deal with. I'm just suggestion them in case the thought is too powerful and the only way it could be added is to have restrictions.
A1) It is a ViP perk. (maybe even allow ViP to earn full XP on the secondary?)
A2) If you are not a ViP, you can buy an item out of the store to "mark secondary".
B) You must be a level 25 character to allow "Secondary".