What A and B actually leave you
The Part A hospital deductible arrives per benefit period, not per year, so two separate hospital stays can mean paying it twice. Part B covers 80 percent of approved outpatient costs, and the remaining 20 percent has no ceiling. A serious diagnosis with ongoing treatment can turn that 20 percent into tens of thousands.
Add what is missing entirely: prescriptions, routine dental, vision, hearing, and long-term care.
The unlimited-risk problem
Most insurance protects you with an out-of-pocket maximum. Original Medicare alone has none. It is the only major coverage Americans carry with no cap on a bad year, and that single fact is why most beneficiaries add something on top.
The two fixes
A Medicare Supplement plan keeps Original Medicare in charge and pays the gaps, trading a monthly premium for near-total predictability. A Medicare Advantage plan replaces the delivery system, caps your annual spending, and usually bundles drug coverage.
Going bare with A and B alone occasionally makes sense, mainly for people with strong retiree coverage from a former employer. For everyone else, the gaps are the plan-shopping starting point, not a detail.