Not sacrificing Toshiro would not be a plot shield. A plot death moves a story and character forward, at this point neither Bleach nor Naruto has any characters left that's death would affect the plot with anything but cheap sorrow. Some readers may like that but the majority do not.
Toshiro's death would make him cannon fodder, a cheap death for cheap shock and drama that would lose Bleach even more fans and just end the Manga on an even more dismal note.
A death should move a plot or character, honestly Yamamoto is the only character in all of Bleach whose death had any meaning to the plot. Bleach is not constructed to have plot driven deaths. All the other good guy deaths, including Gin, that have happened have been nothing more that cheap body pile building deaths that have done nothing for the manga than put it in the sorry situation that it is in and will likely end in, if not worse.
