Replacing welfare with UBI isn't going to happen, we will end up paying for both.
When my family was on food stamps, if we mismanaged our money and food stamps one month (spent them too inefficiently, accidentally let some food spoil, or such), then, yes, society would absolutely have let us starve (though not in the streets, as we did own our house and were in a rural area). We might have been able to get some further help from family or friends, but not from any form of welfare.
Yes, just as much as we do now if they resell (e.g., for drugs) or squander their food stamps. Or, for perhaps a more perfect parallel, the same as the existing system does for anyone who had the income or assets to fail to qualify for means-tested benefits in the first place but fails to use them to meet their basic needs.
> Again, you're not seeing the fact that welfare often comes in the form of services, which the welfare receipts can't waste.
The things that directly stop people from starving in the streets are cash aid and aid that purchases food, neither of which are proof against recipient waste.
There are other forms of means-tested aid (largely, by dollar value, Medicaid) which are harder to misdirect, but UBI plus a default (with premium) public health insurance plan that you can opt-out of the purchasing a qualified private plan (a no B.S. individual mandate rather than a pay-a-small-fine one) handles that.
> I guess someone could try to seell soup they get at a soup kitchen
Soup kitchens aren't means tested public welfare, they are generally private charity, and are usually means-tested only in that people with any other options don't go.