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 That's how I see it, at least, and in that context it is not outdated or ludicrous at all.
Your opera analogy breaks down in light of the way opera is often performed. Many modern opera companies (even world-class ones like the Lyric in Chicago) got bored with traditional costumes and sets a long time ago. I've seen Pagliacci
performed in a post-apocalyptic wasteland and The Barber of Seville
performed in a dream world where furniture floats near the ceiling.
Next, I don't think anyone in this thread is arguing that anything is outdated or ludicrous. The question at hand is: if you take certain things away from aikido that are not part of the physical, technical practice of the art, is it still aikido? And when you say, "Opera is opera with or sans attire or opera hall," it sounds to me like you think the answer is yes.
In regards to your specific point about terminology, I think most activities that are widely practiced internationally have a language that serves as their international medium. Russian tennis players know English tennis terminology, German fencers know French fencing terminology, and I, an American musican, know Italian musical terminology. International terminology is a useful thing to have. We keep it around, I think, because it's useful, not because it is sacred or essential.