It would be interesting to know what low strength people usually die from. One example where muscle mass would reign supreme would be in fall related deaths. Strength helps balance and helps prevent a fall in the first place. It also helps catch a fall. Strength training also strengthens bones so that would help you avoid a broken hip. Then if you do break a hip, your extra muscle mass and strength will deplete but not to the point where you're now bedridden. It's money in the bank.