The point of locking down everyone is to slow down the spread of the virus through the entire population, which protects the high-risk people from accidental exposure, until a vaccine can be developed and deployed. That way we get herd immunity without killing the percentage of people who can't survive a full infection, but will be just fine with a vaccination.

Having healthy people isolated is not about protecting the healthy people. It's about stopping the healthy people from spreading the disease to the high-risk people.