Don't you think they would have interbred?
I think that their vanishing has to do with a lot of factors, like the ice age ending, disease outbreaks, competition, and such. Interbreeding may very well be one of them. Like I said, you can't dismiss the idea just because one study was done on it.
Of course, maybe Neanderthals found the concept of mating with homo sapiens to be appaling. Who knows? =P