Part of the benefit of UBI is removing a large amount of overhead that comes with current welfare systems. This is done by writing a check to every citizen in the system with overhead only to confirm that one citizen gets one check. When you add in other checks you increase how much money needs to be spent on just running the system
If I literally see a dime from the UBI check and pay $2 in taxes to support the UBI program for every dime in UBI I receive, it's reasonable to say that I'm not really receiving UBI on a net basis.
(I still support it, and your arguments for why it must be universal are congruent with my own thinking.)
That's completely true, it's just cheaper to send the same check out to everyone than pay for the administrative costs to figure out who gets the check that week.
A UBI alongside a progressive income tax is identical to a negative income tax, assuming that the negative income tax is truly progressive (that is, the marginal tax rate is monotonically increasing over the entire income range; I only add this caveat because I've seen NIT proposals where the marginal rate was higher in the range where the total rate was negative, then dropped down, and was progressive beyond that point.)
That is, with any such NIT, you can add a fixed amount to the tax laibility so that the amount is never negative, and add a UBI of equal amount, and get exactly the same net results, and with a UBI you can subtract the UBI amount from everyone's tax liability, delete the separate UBI, and have an NIT with the exact same net financial effect. They are basically interchangeable.
Administratively, they are different, in that the UBI payments are effectively an advance against the potentially negative income tax that those above the point with 0% marginal tax have to pay some or all of back at tax time. But that's the only difference.
I wouldn't see a dime because I make far, far more than the people who need it. I don't need help.