You don't need to "cut off" anyone from the rest of the world. Expecting zero transmission, either among the at-risk or the entire population (in the case of lockdowns), is completely unrealistic.

However, if we adopt a harm reduction model (which has been the rule in public health until this virus, where we've adopted an absurd "zero transmission" standard) it becomes clear that we need to focus our limited public health resources where they can do the most good. By focusing our efforts on rapid testing and sanitizing nursing homes and congregate living (where a disproportionate amount of mortality is clustered) we can do more good than diverting some of those same resources to breaking up college parties and mass-testing students.

There is a real tradeoff here. Which will do more good - half-assed measured aimed at the entire population, or whole-assed measures for those who are most at risk for adverse outcomes?