There are certainly UBI proposals in which at some point UBI does scale back. I don't see a good reason why it scaling back would disqualify it from being a UBI, and I certainly see little likeness to unemployment insurance.
I guess the justification for not scaling it back would be that it makes it more palatable for the people paying for it. If someone received $10,000 in UBI and payed $10,000 in tax it would be neutral but their attitude to other recipients might be different.