The Avengers couldn't save anyone that wasn't the dust people. Russ would know that if he was paying attention. The point of going back in time to get the stones rather than just preventing Thanos from snapping his fingers the first time is that the stones are going to be used in their present timeline to bring back the dust people. And they couldn't being back anyone else who died as the Hulk showed by trying to get Black Widow back unsuccessfully when he snapped the dust people back. Vision stayed dead too because he died naturally before the snap. If they did try to interfere in the past to prevent deaths that would just create an alternate reality which would create a myriad of different timelines that would not being them back to their present timeline.