If we got UBI today, the majority of people would stop pushing for the better system and would be content with "better than they had".

Effectively setting back liberation by centuries and perpetuating suffering, meanwhile it would always be a sword of damocles dangling above the peoples head with fear of it being withdrawn or diminished by future conservative governments at any minutes notice.

And frankly arguing for it is a terrible idea on principle, it's like haggling and starting from the middle instead of the lowest.