It's very simple. Not all people need the same amount of resources to live. A healthy college student needs very little, a sickly elderly person needs a lot. The complex social support system in place today attempts to redistribute the tax money allocated for social services in the most fair way possible. The negative income tax either replicates this complexity (but then what's the point) or the negative income tax seeks to abolish the social support system in order to redistribute money to the rich.
Now as a proponent of the negative income tax it should be on you to show how it could work, not on me. But whatever, I can do some numbers. Since you mentioned you wanted to get rid of social security the negative income tax has to be at least that (or the elderly will have to eat dog food like they did before FDR), and social security is about $30,000 in 2017. There are 240 million adults in the US, 240 million * 30k = 7.2 trillion. That's the same as all local, state, and federal government spending put together. Whoops! So clearly, there is not enough money to give every adult citizen the equivalent of social security. So what are we going to do now? Means test the negative income tax so the elderly get more and young healthy people get less? Means test it so disabled people get more? So that single parents get more? Is the negative income tax just going to replicate the entire social support system it has just senselessly abolished?
If you want to think clearly about the negative income tax think about which demographics end up paying more and which demographics end up getting more. If the goal is just to redistribute wealth from the rich to the needy, you can easily accomplish that by adjusting the income tax brackets. So clearly negative income tax proponents don't want that. Is the goal to increase efficiency? No, because social security has less than 1% in administrative overhead, and negative income tax supporters still want to get rid of it. So clearly that's not the goal either. Is perhaps the real goal to demolish the entire social safety net while giving tax cuts to the rich? Why yes, it is!