What you are describing are in fact the freest parts of the health insurance market.
Employers, quite rationally and of their own free will, choose to offer health insurance as a benefit because it attracts and retains talent better than other similarly priced incentives. The fact that you're hesitant to take a leave reveals that it's working.
Insurance companies, quite rationally and of their own free will, choose to offer steep discounts to purchasers who buy for a large group of people instead of just one or two. It reduces their marketing costs and gets them a better balance between sick and healthy clients.
You, quite rationally and of your own free will, choose to both accept your employer's offer of discounted insurance and to remain at your position longer than you would otherwise prefer to maintain that insurance coverage. In fact, the only thing you foresee changing the status quo and allowing you to retire at all is when you become entitled to benefits from the state in the form of Medicare.
Nothing in the phrase "free market" implies the resulting equilibrium won't conspire to limit an individual consumer's choices.