I then apply it to the context of someone I know (late 80s, early stage COPD, generally frail) who is in a small care home, well out in the country. There isn't the space in the home for staff to have accommodation
I agree that perfect shielding of those in care homes is sadly not possible. But with the greatest of respect, this shouldn’t shut down the argument.
When comparing “lockdowns vs shield the vulnerable”, the question then becomes one of balancing the negatives on each side.
In favour of lockdowns is that it gives more protection to someone in a care home, already above the average life expectancy, to have the chance to continue living a further few years beyond average life expectancy. (Note it doesn’t give 100% protection, the virus still spreads and the risks you described are still there, the benefit is incremental).
On the other side of the scale are the countless harms lockdowns definitely do, primarily concentrated on the young. Indeed they degrade the quality of life for many young people to something tantamount to being in a care home. Some will commit suicide, and every one of those counts for at least a hundred care home deaths based on a quality of life years calculation.
I do accept now vaccines are actively being injected into the elderly and care home workers, the calculus has changed. But that won’t take long, and if there are any setbacks the above argument comes to the fore again.