"Democrats promising $30K BI? Well we're gonna do $35K!"
You keep referencing ACA, but there is currently a new bill to repeal it: https://www.lgraham.senate.gov/public/_cache/files/06822f49-...
Even if we guarantee some sort of income floor, there will still be those who, despite that guarantee, need additional assistance. Whether it's because they have more children than the average, are irresponsible with spending, are victim to a drug habit, unexpectedly lose their jobs, or succumb to medical debt, we have to come to grips with the idea that UBI won't be a cure-all.
Even if we acknowledge (and not all will) that most people who are dependent on safety-net assistance are perfectly good and responsible, depending on what the amount is, what their life circumstances are, and a myriad other factors, I don't think we can take it for granted that UBI obviates those specific-purpose programs for the remainder of those who need assistance and for whom UBI is not ideal.
Put bluntly, there are people who will take the entirety of their UBI check and spend it on stupid things, and end up without money for food, or for their childrens' health. What are we to do about them?