I've seen high level play of other rhythm games before and I still found the Stepmania segment to be entertaining. I'm glad other the other exhibitors and speedrunners were able to as well.
It's great that they can keep in perspective why the audience might find something entertaining. Its even better when speedrunners, even though they themselves have done what the current runner has done thousands of times, aren't completely drained of enthusiasm. Sometimes they even take it further by expressing with lots of emotion why these regular occurences are indeed still special instead of being a killjoy like you and others in this topic.
The Stepmania segment isnt taking anything away from other rhythm games. In fact if it weren't for that segment and the guys being enthusiastic about the Brain Power song I would never have comes across these guys playing one of the rhythm games you think is being snubbed:
https://youtu.be/6r6fn7YBJcw
Eventually I hope you'll realize that for most of the audience, none of these runners are stealing the spotlight away from similar games, that they are in fact doing their part to get people into each of these communities. The unfounded hype and praise you all think Stepmania received is helpign to ensure people keep watching and discovering more games. There isn't a better way to get the audience excited about a scene or genre than to showcase skillful play of something the audience has already played and intimately understands the basics of already.
Also you really don't give enough credit to DDR and what it's did for rhythm games outside of japan.