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.
