The only way for this approach to work would be to define the metrics of "success" before starting this taper process. Also, it would need to be implemented in such a way that makes the process unstoppable.

Otherwise, it will be nearly impossible to taper "off" in the event of a failure.

It's easy to start giving people money that they never had, but once they have a taste, it's going to be hard to take it away.

I also think there's never been a pattern of having "success metrics" tied to laws. Laws are LAW. The best we could get using current legal technology would be a built-in expiration of the law. And if the people think it's good, then we'd vote for representatives who will retain it.

You lose support from many people if the plan doesn't include dismantling massive government agencies responsible for other support/giveaway programs.

You can't dismantle those until you're at the 90+% range of any reasonable UBI payout.