I think that for the most part, Marines can pretty much replace old organs, and useless parts with the technology they have. So at least for the most part, Marines can last. Though, some marines like that apothecary mentioned in Salamander sometimes get a little tired from some aspect of war. In the apothecaries case, he was pretty pissed that so many men, women, astartes had died and he was pretty sad he couldn't save them. He probably along with most of his company have regrets that in neither case of their captain dieing they could save them.