Given the argument that you want to make, I do not see anything wrong with this thesis statement. I really like the way that you structure it -- how you have the bit at the start about what proponents think and then you transition into what you think is true.
If you wanted to add one more sentence, you could put something in about the waste of money that you mention in the question. I would say something like
This means that welfare programs spend large amounts of taxpayer money while producing results that are the opposite of what is intended.