It won't, though: at least, not in the way that 'herd immunity' is commonly understood.
We technically had herd immunity from measles, but it wasn't effective at stopping people from dying until there was a mass vaccination program.
If re-infection with covids19 is possible, then herd immunity won't occur.
Herd immunity isn't a binary thing. Outbreaks reduce as herd immunity increases. Nobody is saying covid-19 will go away thanks to these measures. Indeed, the government have been saying it'll probably become a mild seasonal illness much the same way the Spanish flu is now.
Remember our species has only eradicated one single infectious human disease in our history, Smallpox, and it took hundreds of years. Coronaviruses are notoriously hard to vaccinate. Locking down society indefinitely (which itself had a human cost) based on the hope we'll eradicating covid with vaccines seems like wishful thinking.
Scientists are saying a vaccine is at least a year away and this disease is beginning to spread through India and Africa.
What is the alternative plan here?
Moreso than you'd think. The effect of some percentage of the population being immune is to reduce the effective R0 (transmission ratio). At an R0 of less than 1, the disease cannot sustain community transmission and dies out. Above 1, you get exponential growth. This is a stark threshold. Usually you need a population that's 60-80% immune to achieve this threshold.
As herd immunity increases, the R0 decreases. A disease with a low R0 spreads more slowly (hence the logistic curve) and is therefore easier to track and trace manually, and has a lower peak and total infection count.
I don't think this is binary at all, or even _that_ stark of a threshold. Transmission doesn't continue at the same exponential rate until it hits around 60% and then suddenly disappear. It doesn't even come close to approximating that behaviour.