At least one FULL bathroom on the first floor is ideal, even if the home is multi-story with an elevator (elevators aren’t fail-proof). Regardless of whether or not you currently need one-level living, a bathroom on the main floor is smart. Don’t forget friends or family who might have difficulty with stairs.
None of us can guarantee that we’ll always have “perfect” health. Even a short term injury can make a difference in how we perform our daily activities. If someone is recovering from an illness or injury, a bathroom without steps to access is ideal for taking care of needs. A safe shower is better than a bath at the kitchen sink any day.