You don't believe more automation and AI use will result in reduced available jobs overall?
We shouldn't treat this as a question of pure believe and speculation. Currently it is not happening. If anyone thinks otherwise he has to present valid data (that goes beyond single indices or anecdotal evidence of a sector). It might happen at one point in the future, but that could be in 10 years, 50 years or 500 years. We are terrible at predicting new developments, that aren't already occurring. Genuinely terrible. Nobody has a good track-record at doing so. Its fine to speculate, but one should understand that nobody can be confident about his predictions and that's crucial. Its difficult to justify major political or social reforms based on speculation.
There are various false assumptions about "jobs", that are commonly hold by many people. The most obvious one is that "there is a fixed amount of jobs". 50 years are migrants were blamed "to take our jobs"(well, sadly people still do that) and now apparently the new version is to claim robots are doing it. When ATMs became widespread available, the amount of bank-tellers increase (against any common sense). Thats just one example, but the history is literately just a series of "new unexpected jobs got created, while other vanished". There has never been a development that created permanent technological unemployment. I am not claiming that this is never going to happen. I am just saying that it isn't a valid argument for UBI, because its not happening at the moment. I also really struggle to see how the numbers of medium and high-skilled jobs could be limited in any time-frame that I can reasonably think about. Technological advancement always changed/increased the requirements for jobs. Politics should talk about the education system. Thats the only way to address this trend properly. Yet the education systems shouldn't be simply expanded (more money, more years), but the structure of them has to change. Any other attempt to deal with this is futile.
I also want to repeat that I am actually somewhat in favour of a well designed UBI for other reasons.
I think a very important question is about what people consider to be fair. Currently developed welfare systems try to micromanage outcomes and they do that for good reasons. It decreases the likelihood, that someone is "totally fecked". Thats why there are a billion different rules which support you get under which conditions. Once you abolish this system and lump (most of it) into a single payment, the range of outcomes will increase. Some people (who are lucky or make better choices) will end up in a better place, while others, who are not lucky/smart will suffer. Even I am quite uncomfortable with that. Thats why I don't think UBI should replace all other social programs. Yet once you are making exceptions (e.g. for disability), UBI becomes a lot more like the already existing social welfare system. The one exceptions is that it would abolish all the "push" factors for unemployment benefits. To discuss if UBI is better than the current system one would need to outline at least the basic structure of it. Some UBIs would suck, while others would probably improve the situation.