"Let the young people go to work" means you now have to somehow completely isolate everyone over 60... while still providing them health care from uninfected people... while almost literally everyone else in the country is getting infected.
And even then, 3% fatalities under 60 with a 2% fatality rate means that 0.06% of those people under 60 will die. In the US, that means a minimum of (napkin math: 327 million * 85% * 0.06%) 1.66 million people, and almost certainly much more than that, because all of those under-60 severe cases are wildly overloading hospitals and so are unable to get the care they need.
All of this is the strategy that the UK is currently undertaking, which can be generously described as "completely insane" because they don't have the support system for all of those over-60 people to safely self-isolate or the hospital capacity to handle the flood of severe under-60 cases that will show up over the next month or so.