The easy way to do this is to give people a one-time election between social security and the basic income, so that nobody can object that they're not better off. Anybody who isn't at or close to retirement will choose the basic income because it begins paying immediately, the lifetime payments are higher and they wouldn't have to pay social security tax anymore. Then in 20 years or so there would be no one left on social security and it could be discontinued.
> Similar problems with Medicaid, it's worth a lot more than $3,000 to the people that are getting it (mostly because children).
It seems like your argument is that we can't do it because there are a relatively small number of people who would be better off under the existing system. But by that logic we could never change anything, even if the net benefits are very large, as long as there is anyone who is better off under the status quo.