What is believable?

It is believable that the young and healthy, less so the old and healthy, the invincibles, think they are too busy to be bothered or health insurance is unneeded, especially when they can sign up when they need it, even if it may not be right away. It is believable that they find the cost very expensive for something they don't expect to use. It is believable they would ignore it until they need it. On the basis of these, a penalty can be motivating, at least if the penalty is significant enough relative to the cost as it would be for the highly subsidized. Even if the penalty isn't significant, it can still promote rational decision making.

Health insurance is a transfer from the healthy to unhealthy. It is never a good buy for the healthy who can only thank not needing it, while it is beyond value for the unhealthy who can only thank the healthy for making their treatment possible. Not only are subsidies subsidies, and insurances are subsidies, but prices themselves are subsidies, priced to collect enough from those who can pay to cover those who cannot as well as fund research and return profits. Those in serious need will never be able to pay for it themselves but always must rely on others, whether through pricing, insurance, subsidies, or direct provision by government. Direct provision can be the most efficient as long as prices are negotiated between monopolies but suppliers don't want negotiation while government doesn't want to pay. This leads to the variety of indirect schemes.