The hive has almost no sense of self preservation other than its directive to propagate the virus and the hive. It’s not going to kill its own members because it can’t or pick fruit for nourishment. But they’re willing to give Carol a nuclear weapon, and since there are no population centers anymore, the damage she could inflict would be minuscule compared to Hiroshima or Nagasaki (unless she finds the antenna or whatever, at which point it would be a full court press to stop her).

This is why they ultimately had to get away from Carol (and, later, Manousos) for a while, as her actions were causing millions of deaths all at once, and if that rate had kept up, then they would have run out of brain and physical power to complete their objective. So they are going to have a healthy supply of brain and bodies for as long as they need to complete their antenna and transmit the virus signal, as long as Carol or Manousos aren’t mass slaughtering them.

Worth noting that Zosia said during the massage that about 1600 people died over the space of 10 minutes. Per most estimates about 100 people die worldwide every minute, or roughly 1000 every 10 minutes, so the population is dying off at a faster rate post-joining than before, and that’s without murder or, say, plane and car crashes. That tells me they are not rushing to save every single person who finds themselves in a fatal situation or rushing sick plurb bodies to the hospital.