While there's no denying that capitalism can be ruthless and unrepentant, and what we have for capitalism currently actually is that, calling the progress of capitalism "ruthless" and "unrepentant" is just as loaded as calling UBI communism/socialism/etc, and leads to needlessly heated debates and ideological headbutting. It fails to convince anyone of anything.
Even if capitalism isn't ruthless and unrepentant, the value of human labor goes down, and isn't necessarily undermined, when capitalism seeks efficiency. There is no strict requirement that capitalism be ruthless and unrepentant—those are, in fact, human traits that are impossible to apply to the unthinking and unfeeling system that is capitalism.
When capitalism replaces human labor with machines, it isn't to undermine the value of human labor, it's to be more efficient. Ultimately, the result is the same, as you describe: humans are unable to exploit their labor for even basic living. But calling out capitalism as some kind of evil hydra that explicitly sets out to destroy the lives and livelihood of the working class is unproductive. That is an unfortunate side effect of capitalism, and of capitalism gaining increased prominence during an age when human labor itself had high intrinsic value but was also relatively abundant. It is a side effect which must be addressed.
I find it much more effective to couch UBI in terms of the end result of capitalism: it makes no sense for a capitalistic system to use relatively inefficient humans to do work that machines can do faster and cheaper. To ensure and give inefficient humans jobs that capitalism could otherwise find ways to do sounds more like a socialist makework ideal than UBI does. It is actually in capitalism's best interest to pay people to stay out the areas where they can not perform tasks as well as machines can. This frees up people to find other avenues of contribution, perhaps finding further efficiencies, without using up all their time and energy leveraging their faculties purely to stay alive.
