This is a great comment - I'll just make one minor but important correction. Reserve requirements are fairly uniform across states (see [0] for a list of differences), so they likely wouldn't be a major driver of health insurers' decisions about where to incorporate.
The reality would be worse than you describe - rather than being driven by reserve requirements, the race to the bottom would be based on differences in the level of coverage that insurers are required to provide. Healthy people would buy coverage from states with lax requirements to reduce their premiums, while less healthy people would buy coverage from states with more stringent requirements to ensure their treatment would be covered. Premiums in states with stringent coverage requirements would increase, pushing more and more healthy people to cheaper alternatives, and eventually making coverage in those states unaffordable for the people who need it.
The reality would be worse than you describe - rather than being driven by reserve requirements, the race to the bottom would be based on differences in the level of coverage that insurers are required to provide. Healthy people would buy coverage from states with lax requirements to reduce their premiums, while less healthy people would buy coverage from states with more stringent requirements to ensure their treatment would be covered. Premiums in states with stringent coverage requirements would increase, pushing more and more healthy people to cheaper alternatives, and eventually making coverage in those states unaffordable for the people who need it.
[0] (PDF) https://www.naic.org/prod_serv/SPD-OPS-19.pdf