Speaking as a former classical voice student, I can assure everyone that opera is still opera, even when it is watched -- and even performed -- outside of an opera house and in casual clothes.
You don't have to be a fat lady with a shield and horned helmet?!
There still is a universal terminology for music, and a subset of that for opera, no?
The terminology, and music itself, allows opera singers and musicians from around the world to speak the same language, whether their native tongue is Japanese, English, Khmer or Italian.
But, would you perform an Italian opera in Nordic attire? Or Wagnerian works dressed as the cast of "Carmen"? Wouldn't something be missing, graphically, if you just did these works in street clothes? How would you be able to tell who the characters were, or where they were, or their stations in life? Opera is opera with or sans attire or opera hall, but it's even more so when it is dressed in the trappings that its composers had in mind when they wrote the pieces.
With martial disciplines, we can learn just the physical skill set, or we can incorporate a cultural and historic context to what we're doing as a way of stepping from the workaday world into a training environment in which our adherence to rei, specialized terminology and dress become a form of meditation that enhances our focus. That's how I see it, at least, and in that context it is not outdated or ludicrous at all.