Bluntness not only is herd immunity a dangerous fallacy as we can't actutally lock the vunerable away (see the tragedy of agency workers being unwitting typhoid Marys in care homes for example), there is no proof that immunity is retained for a long time and the best estimates I have seen suggested it would take two years for everyone to catch it.
Lockdown is not about protecting the vulnerable or the small number of 'healthy' people who would die. It was about preventing the NHS being overwhelmed. If we'd just let it run rife, the evidence suggested that hospitals would stop functioning, the healthy and the vulnerable would die in much greater numbers as they wouldn't have been able to treat as many people. Non-covid patients wouldn't have been able to access treatment for much longer and more non-covid deaths would have happened.
Economists were also advocating lockdown because they knew the economy would've suffered anyway. In Sweden, where people were not locked down, they have stayed home anyway, not fully, but life is far from normal. If hospitals were overwhelmed here and death rates higher, most people would feel less safe and less inclined to go the pub or the hairdressers etc. Large numbers of people isolating from work would have been a big problem (and still might be).
I agree that we need to get out of lockdown, but do it in a way that makes the sacrifices people have made worthwhile. So hang on a bit longer till the r0 is lower, open up places in as safe as way as possible, keep with the working from home where possible, admit that opening school is about essential childcare (and vulnerable kids) and prioritise places accordingly, get track and trace established, make face coverings compulsory in crowded indoor spaces.
It's not lockdown until vaccine vs open up and to hell with it - there's a middle way.