You do realize that in order to reincarnate say my fighter to a savant I then need the gear to level a savant from 1 - 28? I have no interest in heroic reincarnate most of my toons - if I do so I generally add 1 or 2 in the same type of class for the ability points. I just don't feel like re-gearing in a completely different class. Both because I don't have the space but not really any interest in spending that time since I do not have any interest in any type of completionist.
So right there is one of the most silly aspects not only of your argument but that of the design, where the 'best' solution to a terrible design idea is to re-level from 1-28 in a completely different class forcing a player to re-gear in order to be able to synergize with the sphere.
Now here perhaps the idiot proof solution.
Karma should not be tied to a specific sphere. 6 million points is 6 million points. 6 million Xp is 6 million xp. All type of grind have one thing in common. TIME. Time to spend. All assumptions from a dev should be 'if I want to have people play this system for as long as possible giving us time to come up with a new thing to do in the end of it, what is the common denominator?'
The amount of time I spend leveling A to Z.
So why do I need to make that hamster wheel terrible by doing it in a way that makes it harder to do so because there's no synergy?
BTW - there's absolutely NO challenge in playing EN from 20-28 - there is a lot more challenge in playing EE from 20-28. So your 'challenge' is actually a complete lack thereof.