Earlier quoted context omitted.
It seems that you are confusing marginal tax rate and effective marginal tax rate. UBI and NIT (Negative Income Tax) are designed to increase incentives. That's why free market economists like Friedman fiercely supported it. Milton Friedman - The Negative Income Tax: https://www.youtube.com/watch?v=xtpgkX588nM
You can insert the word 'effective' into my sentence above, and it remains true. It is entirely possible to design tapered benefit withdrawals without resorting to unconditional basic incomes. (It's also theoretically possible to design tax brackets for a UBI/NIT system which imply effective marginal rates at 80%+, though I'm not sure why anyone would actually want to do that)
Tapered benefit withdrawals are the exact reason for high effective marginal tax rate. You can't use them.
You can design welfare system effective marginal tax rate decreases smoothly, but you can't make it linear. (unless you make the taxation incredibly complex and effectively same as UBI/NTI)