Board Thread:Manga/@comment-27321453-20180508205459/@comment-24407809-20180508221635

Because there isn't any value in those characters. Ymir was a red herring who was only prominent for around 10 chapters, and all Sasha has done for 80 chapters now is stand in the background and say nothing. Could Isayama have given them more prominent roles? Yes. Did he? No. And as a result, neither of them hold any real value, and so the series loses nothing with their deaths.