I can't imagine anything short of a nuclear war causing US GDP in 2050 to be lower than it is today. Maybe some of the more hysteric predictions of climate scientists would put climate change in that category too, but it looks to me like that 2052 prediction is for declining GDP by that year, not that it'll have plummeted back to present-day levels. Personally I think we'll be very lucky if climate change is the worst thing we have to worry about thirty years from now.

What has people in the tech sector worried about unemployment and advocating for UBI is technological unemployment - as new advances in AI and robotics  come along and displace workers, there's a hotly debated question of whether those workers will be reabsorbed into the economy like in previous waves of automation, or whether this time is different. When self-driving trucks replace regular ones and all truckers are now unemployed, for instance, where do all those people go?

If those fears come true, and there's no UBI, the lucky people whose jobs haven't been automated away yet won't feel the pinch, since GDP will be rising faster than ever. But a permanent underclass will be created, and as technology advances more people will be dropped into that underclass every year while the capitalist class gets richer and richer.

With UBI, all the jobs disappearing while GDP just keeps growing is good news, it means people live more and more comfortably while nobody has to work who doesn't want to work.