1. The intrinsic value of human life, and the fact that "do not murder people when they cease to be perfectly optimal to the economy" is perhaps the biggest Schelling fence in the history of humankind, for the simple reason that there is no earthly reason to believe that the economy and automation do not have the potential to advance to the point where 0% of humans instead of 30% of humans will be economically viable- at which point your logical conclusion must be "let humanity go extinct and be replaced by economically-optimized robots".
2. Modern society itself "flies in the face of evolution". Primitive society flies in the face of evolution. There is a fairly strong argument that sapience itself is dysgenic, as it causes humans to do things that aren't "instinctively optimize survivability". If "evolution" is your core principle you have already rejected humanity, and using "evolution" to support you point is the naturalistic fallacy: evolution doesn't move towards anything. In addition, the current situation isn't the result of "evolution", it's the result of emergent technology.
3. Which is why as we move closer to near-post-scarcity I predict that we will see a drift away from the current economic paradigm similar to the drift that occurred during the Renaissance, with economic power becoming less centralized. UBI is a transitional position, not a long-term solution.
4. Sheer Rawlsian self-interest. Going by random, it is unlikely that a person will fall into the category of "not-outmoded people", and you, specifically, almost certainly do not by the mere fact you are posting on this website.
5. Even if you believe you are in that 30%, UBI is in your best interest for the sheer, simple reason that 70% of humanity will not sit by and let themselves die, and you likely don't want to end up being up against the wall when the revolution comes. The army and police force are in that 70%.
Honestly, the fact that I'm having to give reasons not to kill 70% of humanity boggles my mind.