I haven't read it, but in most cases countries who have universal coverage, participants have the option to purchase higher cost add-on insurance, but not to opt out of universal health care program altogether. Similar to people who have different levels of medicare, but private instead of government insurance. It's not different than what we have now, where someone may have either insurance through their job or through the ACA but also purchased dental, vision, medication plans.
on Canada website: Private health insurance: Private insurance, held by about two-thirds of Canadians, covers services excluded from public reimbursement, such as vision and dental care, prescription drugs, rehabilitation services, home care, and private rooms in hospitals. In 2014, approximately 94 percent of premiums for private health plans were paid through employers, unions, or other organizations under a group contract or uninsured contract (by which a plan sponsor provides benefits to a group outside of an insurance contract).6 In 2014, private insurance accounted for approximately 12 percent of total health spending.7 The majority of insurers are for-profit.