That's the problem distilled to its mycelial essence: imagining a world without Wagner is like imagining The Last of Us without the mushrooms – they're everywhere – he's everywhere!
The important question is what music might sound like without Wagner.
His world of shadows, of continual exposition, of ideas in a constant state of forming, just as his characters are in a constant state of emotional and harmonic flux, doesn't belong only to him.
Richard Strauss or Arnold Schoenberg would surely have written the same music without Wagner.
But they might have found their own worlds of connection without his influence, and languages more original, less in thrall to Wagner's cosmology of ego and expression.
Meanwhile in the early 20th century, a world without Wagner was exactly what Debussy and Stravinsky wanted.
They achieved it as they reacted against his influence as vividly as possible in their own music.
But without Wagner they wouldn't have had the same force to react against. Perhaps their music would have been less clearcut in its mission to escape him.
Be careful what you wish for: a world without Wagner might have ended up being more – well, Wagnerian!
But that's just the start: no Wagner, no Bayreuth, no secular German temple for Hitler to worship at.
Would Hitler have set up a shrine to his favourite composer, Franz Lehár, and his confectionary operettas instead?
Or would the Nazi poison have been even more venomously applied to Mozart, Beethoven and Bruckner?
