Having more time to develop a vaccine or effective treatments is essential, it doesn't have to be herd immunity. Plus in countries where covid19 rate is low enough, social distancing can be helpful for finding and isolating clusters to suppress the epidemic.
The real problem is that waiting until a vaccine is just not even remotely realistic for so many reasons that it's not even funny. We could end up causing more indirect deaths with social isolation than we could possibly imagine, worst case being a huge collapse in the economy results in a large regional or global conflict.
Are there any examples of humanity developing herd immunity without a vaccine?
"How did it end? The most popular theory of how the plague ended is through the implementation of quarantines. The uninfected would typically remain in their homes and only leave when it was necessary, while those who could afford to do so would leave the more densely populated areas and live in greater isolation.
Improvements in personal hygiene are also thought to have begun to take place during the pandemic, alongside the practice of cremations rather than burials due to the sheer number of bodies."