Viruses evolve as well, and the nasty viruses that we could eradicate entirely if we had good global vaccination coverage instead -- for a host of often-silly reasons -- get to explode in numbers in various outbreaks.
Even if a measles outbreak kills no one, it is still a huge increase in the amount of virus in the wild, and that means faster evolution.
I hate to imagine a parent who has been against vaccines (due to being duped by the anti-vaxxers) finding that their own local outbreak was the source for the super-measles virus, which isn't reliably stopped by vaccines and spreads like wildfire, or is more deadly than current strains.