Car headlights run somewhere in the 700-1200 lumen range.
Admittedly they can be really irritating, but if lights with 500 lumens output or more were really capable of semi-permanently knocking out vision or blinding people, the roads at night would be a massacre.
People routinely deal with having very bright lights shined in their eyes. It happens to us all the time and to the extent that it's a problem we instinctively know how to deal with it without being blinded.
Flashlights are very useful. Bright flashlights are even more useful and they're also cool. I've got an 850 lumen monster that I keep by the front door and it's really handy when I need a bright light. But it's just a bright light, it's not an effective weapon unless I swing it at someone.
Even if I caught someone off guard and gave them a good dose in the eyes, they would still be able to function even if they were somewhat impaired by a blind spot. Just like I keep from running off the road when I accidentally catch an eyefull of the sun around the visor when I'm driving and end up with a temporary blind spot in my vision.
It's certainly true that a defender can use a bright light to his advantage in a self-defense situation, but I think it's very easy and very common to overemphasize the effect that it is likely to have.