When you automate production, prices go down. That's something that can be verified empirically, and hardly anybody will disagree. But when you automate the entire economy, prices should go down towards zero nearly everywhere¹ - that's because when capital freely creates capital, all restrictions on production and competition become meaningless.
When you automate the entire economy, salaries also go down toward zero. That's even part of what allows prices to go down.
So, did workers become richer or poorer on the process? It's a 0/0 situation, and the answer to that question will depend entirely on the path both curves take into 0. But UBI makes that all irrelevant by adding a minimum bound on the numerator. For the purpose of fighting the problems of automation any reasonable amount will do, it doesn't matter that it looks too low today.
But of course, there are other goals that one can reach by UBI, and none of the above apply to those ones.
1 - There are obvious exceptions, some things are truly limited, and some things are limited by the government, culture, or other such forces. The cost of those are the real limit that will say whether the UBI value is too low, not today's cost of living.