Young adults interact with people who are not young adults. Young adults that need not be ill might end up in an ICU unit that an older person with a weaker immune system could have used to their advantage.
As long as there is no vaccine this is going to be a huge exercise in resource allocation.
Wouldn't it make sense for high risk individuals to self-quarantine and for the rest of society to carry on? There may not be a vaccine for at least another year. They've never successfully produced a vaccine for any coronavirus, whether that's SARS or the common cold.
High risk individuals are everyone over sixty, plus anyone younger who has risk factors, such an autoimmune disorder. That’s a huge portion of the population that cannot entirely self-quarantine.
Plenty of high risk individuals are doing just that. Even young people with immune deficiencies are at risk and are making these decisions all by themselves. But just like there is herd immunity in vaccinations there is a similar effect at work by simply ensuring that fewer people have the disease in the first place. That ensures that the chances that someone with an immune systems issue or simply an older person is limited and that when they do contract the disease that there is room for care if they should require it.
This is all about getting rid of a huge tsunami of people for which there would be no care. Once the situation normalizes or there is a vaccine it all changes. But right now we are simply utterly unprepared to deal with this at this scale, and fixing that will take time. Even high risk individuals need to eat, and need visitors. Quality of life, especially for people in such high risk groups is already hit hard, the least we can do is to try to make it easier for them to get the care they need should it get to that, at relatively little expense to the rest of us.
As long as there is no vaccine this is going to be a huge exercise in resource allocation.