My family’s most expensive ambulance bill before insurance was $6k, but that was intensive care transport for our newborn. I believe we paid $250 out of pocket for that.
It’s true that having to pay anything out of pocket affects when people call for ambulances.
Also, keep in mind high car ownership rates mean most people drive or are driven to the ER. Ambulances are typically only used when there was on-scene medical care provided, that continues on the way to the hospital, or when transporting a patient between facilities. A minority of Americans use ambulances because they don’t have personal transportation.