The risk to a stable society (the purpose of Social Security, which is part of its name) is essentially zero by raising the cap.

The risk is substantial by either reducing benefits or privatizing. And I know this because the instant the conversation goes down this road the contra agent devolves to this:

I will pick a pocket first, and shoot grandma in the head second, before this nonsense of reducing benefits or privatization. Benefit reduction, a risk of which is endemic to privatization (grandma might become rich, or she might get kicked out of her apartment, who is to say except the glory of the free market), is objectively more unethical than either picking pockets or shooting old people.