It’s true that if UBI was really on the table, this conversation could go in much more interesting directions. Unfortunately right now it’s nowhere near politically viable (at least in America, where these strikes are happening) which makes it a non-solution to the immediate problems facing workers.
I think your guess about the overlap between UBI supporters and supporters of these union demands is way off base, and is serving to distract from the substance of the matter.
The unions should be advocating for fair pay for employed union members, but not for unlimited jobs forever. If anything, the union has money and can provide a salary to laid-off employees.
Alternatively, maybe unions should be advocating for UBI and advocating for their members to do the same, and to vote for the politicians more likely to support UBI.
we have disability and unemployment insurance. Why not have automated-away-job-insurance with addition of government money to pay for retraining programs. Everybody would be better off - society gets efficiency of automation, and the workers gets new skills and new jobs. My understanding is that there is shortage of various blue collar jobs in particular in construction, green energy, etc. And in general, i think society would be better off sponsoring in major part, if not in full, any retraining a worker wants to take, like with some minimal conditions, say 2-4 years in new job before eligible for retraining. We do see how constant upskilling (even just staying up-to-date with new frameworks) is valuable for everybody in tech, employees as well as employers.