What do you mean by safety nets? If you mean economic need-based programs, so something like "if you earn less than X, you get Y for free", UBI generally assumes the elimination of all such programs, so yes, it does assume cutting other programs. In fact one of the goals of UBI is to replace many complicated welfare trap-generating programs with a single non-welfare-trap one, with the advantage of also eliminating the bureaucracy that those need-based programs require.
What I mean is it is no way those targeted programs can be replaced with UBI. If someone is getting 30k of value from an existing or existing welfare programs, cutting it back to 12k removes their social safety net, regardless of the straight cash they get. Worse, if they run out of that, they really have no safety net remaining.
How does UBI take into account mental incompetence, addictions, poor decision-making, bad luck, and bad investments, and where do you draw the line?
Don't a lot of welfare payments have strings attached, with regard to what you can spend it on? UBI explicitly does not, so you can use it for anything you want. If UBI did completely replace the social safety net, and someone gambles/fritters away their basic income and can't get a job to make more, are they allowed to starve (or depend completely on private charity)?
Same way Social Security works - with the exception of federal taxes and criminal judgements, SS payments are largely exempt from garnishment or other actions to seize assets to repay debts. You can't lose your UBI by taking a loan and gambling the proceeds away, because the debtor cannot force you to use UBI to repay the loan.
That makes sense for debts, but how do you handle people who immediately blow their monthly UBI payment and don't have anything left over for food or rent? At least with food stamps, you kind of have to use it on food.
Weekly or even daily payments would cover part of that. You know the amount that should be added to an account each day pretty easily, since it's the same for everyone. If they waste it and still can't afford food or rent, well, at some point you have to make that their problem.
Why do you assume people will get less under UBI than they do now?
As everyone gets UBI, at first glance it might seem that the poor will get a smaller piece of the cake, but in reality a person with a good income will just give it back through taxes.
Of course, some specific people might end up being worse off (like a person who gets $30k of welfare), but on average poor people should be better off under UBI. Mainly because they can work and not lose anything.
