"The system as currently constituted can’t keep delivering those checks indefinitely."
Why not? To fix it requires a modest upward adjustiment in the cap, of the type proposed by Obama. By contrast, privatizing Social Security would be devastating when we have stock market declines of the type we're experiencing now, and even in the best of circumstances would greatly increase administrative costs.
Many people attack regard libertarians as heartless. I don't think that's the problem. I worry more about their stupidity.