Your points are inconsistent with each other: first you claim that no one's pattern of consumption will be affected, but then you move to claiming that the prices of basic goods will change. Those two things cannot both be true: which would you like me to address?

Overall, your main flaw is that you're considering the economy an entirely static thing. But as the demand for basic goods increases, it creates new business opportunities for both incumbents and potential entrants into the market. Capital seeks out profits, and relatively more capital would flow into the provision of basic goods. That's what prevents the basic income from dissipating into worthlessness.