Presumably, the cost-benefit analysis here is being done on a population-wide basis--leaving a large portion of the population unvaccinated lets them act as a r

Presumably, the cost-benefit analysis here is being done on a population-wide basis--leaving a large portion of the population unvaccinated lets them act as a reservoir for the virus, hindering progress towards herd immunity (recall that not every [willing] adult can get vaccinated, nor is the vaccine 100% effective, especially against some of the mutations we're already seeing, so achieving herd immunity is a meaningful public health goal), as well as increasing the mutation rate of the virus.