I like the current level caps. Moving on from lvl 8 to 9 is an accomplish that should require more than just grabbing a quest group and wiping out quests, even with diminished returns. By that level especially, you should be less focused on just grinding up quests and more on accomplishing other goals and expanding your influence.
That the ones with higher caps require more planning and thought to finish means that your character is forced to be more of a mover and shaker (or follower) and less likely that they are someone who just happens into random quest group going to crush X.
Finally, most of these quests are already fairly easily done when you are at the higher end levels for the quest. Allowing even higher levels would just turn them into nearly riskless grabs for what supplies they offer.
That the ones with higher caps require more planning and thought to finish means that your character is forced to be more of a mover and shaker (or follower) and less likely that they are someone who just happens into random quest group going to crush X.
Finally, most of these quests are already fairly easily done when you are at the higher end levels for the quest. Allowing even higher levels would just turn them into nearly riskless grabs for what supplies they offer.