Second, when they do crash, it is unlikely that a shoulder restraint will be the deciding factor in whether you survive. Many crashes have an “all or nothing” flavor to them, with no chance at survival or almost everyone surviving.
This is not actually true. Although I haven't seen the actual statistics, one site estimated that 95% of all aircraft accidents have survivors. Also, many many accidents take place on takeoff and landing (overshooting the runway, landing in bad weather, etc), in which case the seatbelt is indeed a helpful device.
Here is the citing from an article on the Air France crash in Toronto: "According to an NTSB study of 568 crashes between 1983 and 2000, only five percent of passengers were killed; the remaining 95 percent escaped unharmed or without life-threatening injuries."