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helenS A very good point - though it hasn't stopped the better "museum pieces" still being popular even today. I believe even some of the music from operas that were failures in their own time, is sometimes performed now just as instrumental pieces in their own right.
I wonder if with 19c opera, something like that following J.S. Bach's time happened.
He made the Fugue so popular that "everyone" played them; but after his death "everyone" rapidly moved to these trendy new ideas in the very latest Classical era. Then over time, the Toccatas and Fugues started dipping exploratory quavers back in the choppy "hit parade" seas with new performers and audiences, so the good ones are now performed as often as any other popular Baroque or Classical pieces.