However, I do not think we can contain Covid in an isolated age/social group without significant spillover into vulnerable population. We tried this argument several times and it failed again and again
No, but the government decided under recommnedations ffrom SAGE many months ago that all these people were going to get COVID. Given that, we want them to get it as soon as possible, because that does limit the risk to the vulnerable who are able to isolate themselves somewhat.
If the teenager you live with catches it, you're at a high risk of catching it, it doesn't matter if they're going to give it you today, or next month.
However if you a vulnerable person who meets people in groups (such as a teacher / lecturer etc.) then having fewer possible transmission events because all the group members have it at the same time is an advantage.
Also with isolation, actually having the people who are isolating have the virus is again beneficial, so super spreader events that are discovered are again beneficial.
It's not about isolating covid to groups, it's about getting the infections that have been accepted done quicker.
If we don't accept the infections, well we should've locked down months ago.