Setting aside whether a permanent underclass will be an outcome or not - is it not a bit incompatible to simultaneously believe that all jobs will be gone and that a subsistence UBI is necessarily very bad?
The way I see it, if strong AGI really replaces all jobs, then even a subsistence level UBI (by the new post-AGI standards) will be a world with ubiquitous resources and post-scarcity wealth where people pretty much spend their time how they want. Yes, perhaps it might be an "underclass" in the sense that Musk and Altman meanwhile settled Mars with some privatized space colonization, but I might still be orders of magnitudes richer than I am today - so why should I care, except for status games?
It's basically like someone in the pre-industrial age complaining that the industrial revolution will lead to the permanent underclass of people who currently live on welfare in developed countries. But this is surely a wonderful outcome from the pre-industrial peasant's perspective - even poor people in the western world live better than middle age kings!