Since AI abundance is taken for granted, why not just just offer every single person in the world full health insurance?

A dark side of this is that a lot of unethical, deplorable, and amoral people who were otherwise occupied by jobs might suddenly have a lot more free time to make trouble instead of spending 1/3 of their day working for rent and food to survive.

Not needing to work is a neat future that might be ahead of us. But it could unlock some new negatives as well as positives.

Give me one reason why Musk would honour this.

UBI is not going to happen. You're talking about ballooning the federal budget by trillions of dollars a year.