The problem is that you can't isolate the most vulnerable fully. The current lockdown works because it reduces the spread through the wider society, including all the people who are in contact with the most vulnerable. If you don't do that, then we have the Sweden scenario where inevitably there's someone young and fit and infected who brings the disease into the institutions that should be protected, because these young and fit and infected people work there or go there to deliver things.

Alternatively, you need a total quarantine isolation - e.g. anyone working in a care home should live there and can't touch their kids if they go to a school where will be spread because everyone else's parents are not on lockdown. Or the most vulnerable will die.