> It’s inconceivable that another composer could take Bach’s place in that slot. Even Mozart or Beethoven wouldn’t cut it.

I'd pick Mozart.

When listeners hear Bach enumerate what seems like every significant variation technique for a given theme, they can quickly attribute mastery upon hearing that one piece.

But someone with Bach's abilities could just as easily have only imagined all those variations as potential music, then chosen only a few excerpts for a lighter, shorter setting.

Now, imagine this alternative-timeline Bach wrote in a late 18th century operatic form. For each aria he a) picks a fitting style (or mashes up fitting/effective styles), b) imagines the set of possible music based on the character of the text, and c) chooses what he thinks is the most fitting from all those possibilities. (Also, throw in a bunch of hidden musical jokes along the way.)

That alternative-timeline Bach would be superior to the real Bach. Because he came after the real Bach his output includes real Bach's mastery of counterpoint (Jupiter Symphony, Intro to Requiem). Also alternative-timeline Bach can imitate real Bach's style at will (two-part invention at the end of Rondo in A minor).

The only difficulty is that the listener has to listen to a lot of alternative-timeline Bach before they start to get a sense of the scope of his mastery.

Well, good thing the slot we're trying to fill gives an audience plenty of time to hear a variety of music from this composer's output.

Bach++ ftw.