I don't know why you'd have to enforce anything. Money that people otherwise pay to SS would be put into market funds....they can either do that and have something in retirement, or they can do nothing and not have something in retirement.
But it would take many years, I think, to scale the current working population into that and scale them out of the SS contributions. For someone, say, 5 years away from retirement, their situation is completely different than someone, say, 20 years from retirement. So I think any sort of shift like that would have to be done incrementally, over time, to accommodate for the people who are already "invested" in SS. And it would take some amount of time, and some marketing, to get people to understand and re-gear their brains that SS is going away and they need to participate in alternative retirement options.