@Noeuf
There are multiple reasons you cant just seperate the vulnerable even if we ignore that this has made vulnerable many people that we wouldnt usually consider vulnerable. Its not just really old people who would need to be isolated, lots of people got the letter (inc whole families due to one child with vulnerablities) and services would struggle to maintain a normal service. Its easy to think of this in terms of 90 year olds but this virus has made people with asthma vulnerable and killed young people with no previous health concerns
Even if it was just an issue with the "frail" You cant truly isolate people. Say the general population who you have no concerns about getting it are group C, and the isolated person is A. We can tell A to stay at home, but as its a marathon not a sprint, some people will either still need to go out for essentials them selves or will have carers. A people will need to interact with carers, medical staff and possibly wider groups like delivery drivers or family that lives with them
That creates group B who are the "go betweens" between group A and group C who will have no health concerns generally themselves but interact with those that do. Group B is huge. Its the family members that live with person A, its the carers that drop in to do personal care, its health care staff. You cant isolate these people as the number is too large. B needs to keep low levels of disease, else hospitals will shut and they will pass it to A.
Thus if group C has a high level, it will increase the incidence in group b who will give to group A. Think of C being a checkout worker who serves B who is a carer for A. If C is low risk then B is low risk, but if carers are constantly living and interacting with people with it eventually B is going to get it and either pass it on unwittingly or self isolate and leave A without care.
We are all too interlinked for isolation of one group to be effective