Throughout Doctor Who there have been many heroic sacrifices made by characters. However, there are those sacrifices that could have been easily prevented had different decisions been made. While many of these sacrifices are plot contrivances for practical or plot-related reasons, within the context of the stories themselves these deaths could have been prevented.
These unnecessary sacrifices have occurred all throughout the nearly 60-year history of Doctor Who. While some of these sacrifices are short-lived or one-episode characters many other important characters have also made these preventable sacrifices, including many companions and even the Doctor themselves.
Lexa - "Meglos"
Lexa, head of the religiously fanatic Deons, takes a bullet for Romana who was being shot at by a rogue Gaztak.
Romana should not have let Lexa sacrifice herself, as being a Time Lord the results of a bullet wound would not be as severe. Romana either would have quickly recovered or simply regenerated. As shown previously when Romana tried on multiple regenerations before settling on a current one, the process does not appear to be traumatic to her.
Marat - "Planet Of The Daleks"
Marat, a Thal, allowed himself to be exterminated in order to protect the Doctor, Condal, Taron, and Rebec.
Marat's death isn't just unnecessary it actively makes the situation far worse. His death almost causes the death of even more Thals. This is because the memorable Doctor Who villains, the Daleks, are able to track down where the Thals are due to the explosive charges on Marat's body.
Sara Kingdom - "The Daleks' Master Plan"
Sara Kingdom followed the Doctor instead of going to TARDIS and was aged to death by the Time Destructor.
The Doctor would have presumably survived the effects of the Time Destructor or at worst regenerated. While regenerating may have been traumatic for the Doctor and their companions, it still would have been a better outcome than the very human Sara aging to death,
Cyborg 747 - "Sleep No More"
In this Twelfth Doctor episode, Cyborg 747 sacrifices herself in order to protect her crush Chopra, so he can get to the shuttle and to safety.
This was all for nothing as Chopra was killed immediately after reaching the shuttle by a Sandman. While Cyborg 747 could not have known this, it's a shame either she or Chopra had to die in these circumstances. It's a situation entirely brought on by ignorance of what was in the shuttle.
Toberman - "The Tomb of the Cybermen"
The partially Cyber-modified Toberman sacrifices himself via electrocution in order to seal the tomb.
This is a very impressive and noble feat considering Toberman had to overcome cyber programming in order to seal the tomb. However, it was totally senseless as he never would have to in the first place had the Doctor not decided to assist Klieg in order to see what he was planning. Had the Doctor helped Klieg figure out how to open the tomb all the deaths, including Toberman, would not have happened.
The Seventh Doctor - Doctor Who: The Television Movie
The Doctor is forced to regenerate and has amnesia due to Grace's medical malpractice.
This death could have been prevented two different ways. The Seventh Doctor (played by Sylvester McCoy, known recently for his role in The Hobbit) could have checked the scanner before opening the door to the TARDIS, which would have prevented him from being shot and operated on in the first place. Even if he still had been shot, had Grace listened to him to stop he may have survived without needing to regenerate. If he did still regenerate from the bullet wound then at least he would not have had amnesia due to the anesthetic.
John Smith - "The Family Of Blood"
John Smith sacrificed himself in order to become the Doctor again and punish the Family of Blood.
The issue with this, however, is that had the Doctor not become John Smith in the first place none of the other deaths would have occurred. The Family of Blood would have simply died in a few months had they not tracked down John Smith to where he was hiding and killed anyone who was in the way.
Astrid Peth - "Voyage of the Damned"
Astrid disposes of Max Capricorn by pushing him off a cliff with a forklift, and then dies when the forklift falls off the cliff with her in it.
Astrid could have very easily saved herself had she jumped out of the forklift before it fell off the cliff. It would be understandable how she could die if the forklift had been moving quickly, however the forklift was moving so slowly that she could have definitely escaped in time. If she had saved herself, she could have gone on to be a companion to the Tenth Doctor, played by David Tennant.
Adric - "Earthshock"
Adric is killed trying to change the course the freighter is going in order to save Earth.
While Adric does have noble intentions, he fails, changing the course of the freighter. This causes the death of all the dinosaurs on Earth. Had Adric succeeded in his goals both him and the dinosaurs could have survived. This would have changed the entire course of Earth's history, and it would also have saved lives.
Clara - "Face The Raven"
Clara takes Rigsy's chronolock thinking Lady Me could remove it, only to discover she now had to die.
Clara nor anyone else had to die had she not taken Rigsy's chronolock. Had she not made that decision Lady Me would have simply removed it from Rigsy and taken the Doctor's confession dial. Had this been the case, perhaps the Doctor and Clara could have continued to travel together instead of her tragic death and their separation.