An alternate scenario could also be the most at risk populations (elderly, obese, etc.) lockdown, while the young, healthy, low risk populations gain quick herd immunity
For this to work, (a) the young would actually have to be very low risk and (b) they could NEVER contact those in the sheltering groups. Which would be a problem with families with older children or multi-generational households. And schools would have difficulty remaining open due to teachers and staff being isolated in some degree. And the variants that develop in this regime might have been hard on the young - in Canada, much of the current ICU crowd is young, lower-risk individuals.