Herd immunity is how we cope with most diseases, that’s why we (should) all be vaccinated; it protects those who can’t be.
Doing that with a disease itself, a disease we know barely anything about, other than that it will kill some of the people it infects, is one hell of a gamble. Effectively they’re saying it’s ok that tens of thousands of people will die because those of us that don’t die will be immune to it forever more when in reality they don’t know if we’ll be immune to it after recovering. It might work and prevent more deaths in a hypothetical future outbreak but we don’t know that. If a vaccine has been developed prior to any hypothetical future outbreak (which with the amount of effort going in to this right now I expect it will be) then it will have been a completely pointless gamble and those tens of thousands of deaths would have been pointless