Perhaps the Balrog underestimated Gandalf, thinking it could cast him into the abyss, then fly back up to deal with the rest of the Fellowship. Alternatively, maybe it calculated that its best chance of defeating the wizard was to grapple with him at close quarters. Either way, the Balrog falling doesn’t necessarily mean that it wasn’t capable of flight.
I don’t attribute a lot of positive things to social media, but I will give it credit for turning me around on spiders. Arachnophobia scared the crap out of me as a kid. For years afterward, I’d never sit down on a toilet without first getting down on my hands and knees to make sure there weren’t any spiders lurking underneath the bowl. Thanks to years of spiderbro memes, now I generally view them as comrades.