Well, I just had a prime example of why I hate writing replies on my phone. Just lost a few well thought out paragraph due to my phone freaking out... I will try to recreate them to the best of my ability...
I disagree that he is creating an imbalance by collecting skills. As you mentioned he isn't hoarding the orbs themselves, but instead collecting the skills of the other orbs through the skill of his orb. While this is an ability of an orb that requires more slots than any other known individual in the world he reincarnated into, that doesn't mean that in the "dark" world people are not born with more slots.
While I am making assumptions, which very well may be proven wrong later, I find it likely that the orbs were created, and therefore "designed" by someone (probably a deity of some kind). If that is the case, and the orbs didn't materialize naturally and at random, then the orb he has, "world ruler" was designed to accumulate the abilities of the other orbs (without taking the orbs themselves from their user) for a purpose. The fact that it requires more slots, but not exactly twice the (so far as we have seen) "natural" number of slots people are born with, means that it wouldn't require someone who has lived two lives (i.e. someone who has exactly double the number of normal slots). This suggests that people could be born with more, and the orbs is designed for those people, likely for situations that require the usage of multiple different skills at once and by one individual (i.e. in times of emergency, when a "great evil" or "imbalance" arises).
If that is the case, the MC's soul could have been plucked from whatever route it would have taken naturally to be brought to this pair of worlds to fill the purpose of the one who can defeat such a "great evil" or fix such an "imbalance" as it might have been easier than waiting for an individual to be born with more slots...
If that is true, which I certainly admit is a BIG IF, then he isn't creating an imbalance himself by collecting other skills, but preparing to stop or counter something.
Evidence for this is that the orbs don't appear indefinitely, but are shared between the two(?) worlds - which suggests that they are neither created nor destroyed; the abilities collected by the orb are not inherited by the next orb user, but instead collected within the user themselves (if I recall correctly he was still able to use collected abilities when the orb was taken out).
