Thats correct. The vulnerable should self isolate.
In terms of familys....
What is happening with the existing lockdown measures if a child is vulnerable?
If anybody is sick, they need to self isolate for a couple of weeks until they recover. The vulnerable aren't isolated in this existing lockdown unless they were put in a home which is another issue altogether.
What you're proposing is that they isolate the vulnerable from everybody else until it's safe for them to go back out in society which could be months, all alone away from their family until further notice. At least now, families are together and protected.