There have been ~35,500 games played in the post-expansion NHL. Figuring 40 participants per game, that is 1.4 million man-hours of hockey. In that sample, we've seen one accidental death: Bill Masterton way back in 1968. The death rate is 0.7 per million hours of NHL hockey. He is the only player in NHL history to die as a direct result of an on-ice incident, and he'd probably still be alive today if he had been wearing a helmet.
Here are some other rates for the general public, taken from this article:
Bicycling: 0.26 deaths per million hours
Motorcycling is at least 12.5 times deadlier than post-expansion NHL hockey.
Over its entire history, NHL hockey has been about as dangerous as driving a car in this crude measure of deaths per hour. When you consider how many more hours a player spends travelling than playing, it would make a lot of sense to invest in greater transportation safety than on-ice safety. I don't think I need to type out a list of deaths in motor vehicle accidents to make that clear.
And this probably doesn't need to be said: you and I spend far more time travelling in motor vehicles than we do playing NHL hockey. There are bigger issues out there than mandatory neck guards.
What is the number needed to treat for neck guards? For the cost of equipping every hockey player in the country with one, how many motorcycles could we throw into compactors?