sources: 1. https://seekingalpha.com/news/3551823-just-seniors-not-clear
2. [original] https://www.ad.nl/dossier-coronavirus/40-a-50-nederlandse-co...
Vulnerable or not, the rest of the nation is a transmission vector. Also, not all deaths happen among the identified vulnerable groups: "they are very very unlikely to have anything bad happen" is completely false.
Even assuming a 4% death rate (too high for the less vulnerable), that's a 96% chance of not having the worst possible consequences. So I'd say that's "very unlikely" for them.
This isn't the flu, and it's possible for even people in their teens to die (true of the flu as well), but let's not act like it's the likely outcome.
I've seen numbers in the 0.2% range for younger (< 39) age cohorts. That's twice as deadly as the flu (0.1%) but IMO not "shut society down" levels of bad for people in that age group.
And what does it mean to quarantine people? How are they fed? Can they go outside?
You can't get the virus walking down the street past an infected person.