How does that affect issues like this where an increase in overall costs would reasonably be expected to apply to all insurers?

> On top of that, insurance is optional.

More than 90% of the population has health insurance, which is well over the majority required to bring about legislation.

> If there were a guarantee, it would be indistinguishable from Canada (and practically every other country's) single payer healthcare system.

That certainly isn't true. Serious problems with the US healthcare system include AMA lobbying to maintain a doctor shortage, various patent laws and FDA rules that limit competition and increase costs and a malicious lack of cost transparency. None of that would be improved merely by routing the premiums through the government.