I agree with that. You can't have a voluntary social insurance program. We have insurance for that: regular insurance, not social insurance. That makes it a very tough problem politically for any government that wants to add the self-employed. As you're saying, the self-employed who don't want it would be angry. It's a tough one politically.
My guess is that more self-employed people would be interested in that kind of thing. The number of self-employed has increased a lot. But I wouldn't hold my breath on that one.