If a substantial portion remains unvaccinated and so far uninfected, that is a threat for people for whom the vaccine is not effective

Another solution to the "herd immunity" problem would be to deliberately infect those at low risk of severe illness or death, like we used to do (and in some regions still do) with chickenpox. Or just let people in low-risk populations get it and spread it through normal activity as tends to happen every cold and flu season, while vaccinating people in high-risk populations,

Ultimately a vaccine is useful when the risk of severe illness or death is high, because building up natural herd immunity via infection and recovery presents an unacceptably high risk of injury or death. If the risk is low then the vaccine's usefulness over natural immunity is questionable.

People drive more recklessly when they do have a seatbelt

Perhaps, though then it is a question of whether the decrease in severity of harm due to wearing a seatbelt in an accident makes up for the increased probability of occurrence of getting in an accident. Or whether the severity increased due to increased risk-taking and worse accidents. I don't know the answers to these questions because it's outside the area of my expertise.