There is mismatch between content and title of the above post, lal.
Because most of the post discuss how to be free from future suffering, rather than how it arises or even what is future suffering in first place.
In #6, You write that,
So, that is the background for our discussion. Now, let us discuss how one can get to Nibbāna, or to stop future suffering.
If one were to discuss “WHY” future suffering arises, it would be because of any type of birth in any realm. Without the birth of any kind anywhere, one would not have to experience the objects with Aniccā nature.
So, all suffering that comes with Aniccā nature, would cease upon cessation of birth. Becaus it is only with birth as condition can all suffering can arise, not otherwise. Without birth one would have nibbanā and niccā state hence sukha and atta too.