Quarantine isnt just about preventing the illness in the most vulnerable, it's about keeping the explosion if cases at a level that doesn't swamp the healthcare system.

Absolute containment is highly unlikely, but delay can help a ton.

This is exactly the kind of measures that should be taken in the beginning. Italy is far past that: With 9000 confirmed cases they would need to isolate hundreds of thousands of people. For comparison: Italy's entire prison population is 60,000 people. At that point it's just logistically easier to suspend public life over the entire country for a few weeks.