I don't think this would be a bad thing :) Some people will always be immune, humanity wouldn't die out. And it would be a humane way for gradual population reduction. It would create some temporary problems with elderly care (like what China is facing now) but will make long term human prosperity much more likely. We just can't keep growing against limited resources.
The Dan Brown book Inferno had a similar premise and I was disappointed they changed the ending in the movie so that it didn't happen.