Honestly half the original cast should of died by now, the are clear "side characters" who have spoken a few dozen time throughout the whole series and they try and give them a heroic moment like the dude with the bow recently, but he could of been dead this whole time and I doubt many people would of noticed.
I don't think they should have, killing characters for the sake of it doesn't make a story better, I think even if they haven't been the biggest contributors to the story, they carry weight, Taiga took the leading role and cares for the whole tribe, or at this point "nation" but the group of friends from his era along with Tiari are certainly the emotional core. The "dude with the bow" Ren for a long part of the story was in fact relevant, he was the one in most denial, he was the one who had most trouble adjusting to their survival situation and carrying on war, he might even be the one who has had the most character development in the story after Taiga, he came to terms with his present reality and has taken an active role. Arata has been the second to Taiga in terms of leadership of the group and is who is being his right hand now that he is king, Yuka has become a survivor and attatched to the village people. Rikako, Chihiro and Riku have had much less important roles since the story turned their focus to the ancient humans' conflict with the neanderthals, but they are not insignificant when they come to the scene.
I like it that none of them have died yet, even though I think eventually at least one of them will. But when it happens, it will be a turning point in the story, it will be an emotional moment that we will notice, even if they were not the most prominent characters, in the story they are still a relevant thing to their group, and we will notice how it affects them. They are not just characters that can be thrown away in the plot.