Measles is not "completely harmless". For every 1000 children with measles about 2 or 3 of them will die. And this is in developed countries. That's a lot of children when tens- or hundreds of thousands get infected. It's a virus disease, not bacterial, so there's not much you can do other than waiting for the body to fight it. Or not be able to fight it, as it were.
It's also highly contagious, i.e. it spreads like wildfire. The only way to contain it is to keep up a high vaccination rate. Those who don't vaccinate their children will ultimately be responsible for someone else's death(s), whether their own children are fine or not.