I think the fact John and Fanny are happy with each other says nothing good about the character of either (as you point out) but about marriage it only says there is more than one way to have a happy one. In their mutual evils they compliment each other—she is pushy and he is a pushover. Their marriage is not the kind of happiness I would want but I think it is reflective of life that those who least deserve happiness often have it in abundance. I can only hope there is some future comeuppance for their beastly neglect of Elinor & Company. (:D)