"The problem is limited to a few neighborhoods."
Isn't that enough? How many neighborhoods have to be brutalized before it is a problem? This is an example of denial in action.
"I noted no problems. I was told while I was there that in the late '90s that Brussels has succumbed to "white flight" to the suburbs."
Apparently the Belgians "noted" some problems. You might consider visiting the exclusively moslem "neighborhoods" next time you're in Paris and let us know what you note.
I haven't been there so all I know is what I read here and other venues. I'd really like to read first-hand observations.