I disagree with his reasons.
I think the decline in Faust's reputation reflects
--a more general decline in Gounod's reputation
--modern audiences focus on Faust himself as the central character of a bigger story in which Gretchen/Margaret is merely a supporting role used to illustrate Faust's descent into depravity*
--a modern version would show Marguerite dying insane, and Faust raping/seducing his way through life until he dies. Perhaps she only hallucinates being carried off by angels.The actual ending of the opera is too simple, with the good girl being redeemed and the villian meeting his just rewards.
*I have never read Mann's version. Does he have an analog of Gretchen in his story?
Perhaps an awry one: the Faust character, Adrian Leverkuehn, is forbidden to love anyone as part of the deal with the devil. However, an adorable child, the composer's nephew, Echo (formally known as Nepomuk), causes the composer to love him, and as a result (the composer believes), a terrible disease kills the nephew.
The novel has less to do with
Goethe's version and more to do with
Thomas Mann examining Nazism as a (per)version of
Luther's revolution and how both exploited certain tendencies in what
Mann considered to be traits of a German national character.
Nietzsche and his syphilitic fate figure much more prominently than allusions to
Goethe's theme of salvation, which would be hard, if not impossible, to find.