Let me take the arguments on their merits. The stated aim has been to achieve “herd immunity” in order to manage the outbreak and prevent a catastrophic “second wave” next winter – even if Matt Hancock has tried to put that particular genie back in the bottle this weekend. A large proportion of the population is at lower risk of developing severe disease: roughly speaking anyone up to the age of 40. So the reasoning goes that even though in a perfect world we’d not want anyone to take the risk of infection, generating immunity in younger people is a way of protecting the population as a whole.
We talk about vaccines generating herd immunity, so why is this different? Because this is not a vaccine. This is an actual pandemic that will make a very large number of people sick, and some of them will die. Even though the mortality rate is likely quite low, a small fraction of a very large number is still a large number. And the mortality rate will climb when the NHS is overwhelmed. This would be expected to happen, even if we make the generous assumption that the government were entirely successful in restricting the virus to the low-risk population, at the peak of the outbreak the numbers requiring critical care would be greater than the number of beds available. This is made worse by the fact that people who are badly ill tend to remain so for a long time, which increases the burden.
And of course you can’t restrict it to this age group. Think of all the people aged between 20 and 40 who work in healthcare, or old people’s homes. You don’t need many introductions into settings like these for what we might coyly call “severe outcomes”.