In the episode "Children of Time," we learn that in the original timeline, the Defiant was thrown back in time and crashed, and Kira died because Bashir didn't have the medical facilities on board the Defiant to treat her. So the second time around, the choice presented is that either the Defiant avoids the crash and all the inhabitants of the planet cease to exist, or Kira dies once again.
Question: Since the crew now knows about the anomaly and how to avoid it, and since Kira originally died and therefore didn't matter to the timeline of the planet, why couldn't they have just put Kira in a runabout or shuttle and let her head home safely, while the rest of the crew went through the anomaly so that the inhabitants of the planet would survive?
In addition to the comments already made, there is also the matter of Kira's absence affecting the timeline, as it's likely someone else would have taken her station on the Defiant
bridge in the event that she used a shuttle to return to the station. Even a relatively minor change can have significant repercussions.