The reason J. S. Bach is remembered is not that he had a high ratio of outstanding compositions to mediocre compositions.
Conclusion
Bach is remembered because he was such a prolific (productive) composer, not because he had a high ratio of great songs to mediocre songs.
Intermediate Conclusion (so)
It was inevitable that some of Bach's songs would be outstanding and thus survive the ages.
Evidence
He wrote more than a thousand full-fledged compositions.
Evaluate
The author is essentially trying to Solve a Causal Mystery so we can use your Explain Curious Fact mindset.
Curious Fact ... why is Bach so famous?
Author's Explanation ... He wasn't better than average at making hits (his ratio of great stuff to mediocre stuff is ordinary). But he was super productive. He wrote more than a thousand songs, so obviously some of those will be timeless hits that make it into civilization's permanent playlist.
Goal
Since this is Weaken, we can either suggest an Alternate Explanation for why Bach is famous (maybe he invented a new instrument or a new technique or he had really powerful friends).
Or we can hurt the Plausibility of the Author's Explanation, by making it seem like Bach is not famous just because he wrote a 1000 songs.