Nobody is saying you have to wear the same things people wore centuries ago. What I'm talking about is appropriateness. If an event is "formal" you wear whatever is the formal attire of that era.
Opera as "formal" is a social construct, don't you LOVE that Marxist phrase....
Originally they WERE popular entertainment back in the day. Much like rock concerts today. Every lil' burg and village had their opera house, which was like the old barns they convert to community theaters today. No one cared who wore what.
Back when they were written, your average Hans or Giovanni went to La Scala or Der Sofiensaal, they wore whatever.
Who decides what is "Formal"? Further, apparently years ago when silks and frilly French fashions were considered "Formal" someone somewhere made a conscious decision to change styles. How did this come about?