"Fanny and John are equally awful so to speak and thereby get along---a couple with a harmoniously awful point of view who will be there for each other if one makes a false step toward goodness!"

I forget who made the comment about the marriage of the Thomas Carlyles--"How good of God to give Mr. and Mrs. Carlyle to each other, and so make only two people miserable instead of four." I wouldn't wish John on any decent woman (though she might, by her example, make him better) or Fanny on any decent man (though he might stand up to her and reduce the harm she could do). But it's an interesting point you make, that they aren't miserable like the Carlyles; they reinforce each other's bad traits and get along well. By their lights, they are a "successful" marriage. Brrrrrrr!

There's a fanfic if anybody cares to write it: perhaps a vignette of some occasion when John stands up to Fanny and refuses to yield on a point of principle. Ah, well, if he grew a spine he'd hardly be John any longer, would he? ;-)