Comment by saxonww
Are you aware of any health insurance plans in the US which don't have an open enrollment period? I think this is standard across the industry as a check against adverse selection, but most of the information quickly available is ACA-focused, where it's definitely a feature; since open enrollment is extremely beneficial to insurers, I wouldn't imagine them talking up alternatives.
It's true that there is a list of qualifying life events that let you change or acquire insurance outside of open enrollment, but none of them look like "because I don't like my insurer" to me.
There is private (non ACA) insurance you can purchase without an open enrollment period in the states. However they get around it by being able to deny you coverage for preexisting conditions.