Indeed it is. But if the fear of contagion means that people don't undertake economic activities, the results may be worse than a controlled lockdown as people change their behaviour to reduce their personal risk. We saw that happening in March last year, and again in education in the run up to yesterday's announcement - fear and uncertainty were running ahead of the government's ability to persuade people that the risk was manageable.
There are trade-offs, and there is too little data in circulation for us as individual members of the public to be as sure as we'd like to be. But if we're seeing this "seasonal respiratory virus" blocking hospitals like in the worst of flu seasons, but also transmitting more freely and having significantly more serious effects in those who get it badly, there's a bit of Hobson's choice. Government "locks down" (actually, far from a lock down) which imposes one set of costs, or doesn't and imposes another set of costs. And as I've yet to see anyone seriously suggest that the cost of lockdown measures would exceed the ~75k excess deaths that have already occurred despite lockdowns and other measures, my utilitarian view is that measures like lockdowns, distancing, etc. are a lesser evil than the alternatives.
As for the most vulnerable isolating themselves while society at large continues more normally, I've never quite understood how that's expected to work in practice given the minor issue that those in care homes, with acute vulnerabilities, etc. will still be reliant on those in the wider community. Other than glib one liners, I've yet to see anyone articulate how that might work in practice.