You seem to have the purpose of UBI confused with some sort of actual goal.

UBI is the final, desperate grasp for supply side and end user based economies to continue after workers getting replaced by machines becomes too efficient to disincentivize with grants.

The fact is that a post-scarcity world where desperate powers didn't end the whole thing (unlikely) may not need a universal income at all as needs will simply be met with luxuries either public or earned through whatever system survived.