I'm not convinced that John doesn't want what she does every bit as much as she wants it. I think S&S2 portrayed his affecting final interview with his dying father with psychological realism. He's obviously somewhat taken aback by his father's request, and it's obvious that his accession to it is something that he himself would not otherwise have thought of. This seems to me to be entirely consistent with the book: "he was affected by a recommendation of such a nature at such a time", implying that it took the last wish of his dying father to arouse those feelings which were not naturally his (he "had not the strong feelings of the rest of the family" - Ch.1). This reversal of his natural inclination lasted "for many days successively" (wow - what fortitude!) before he reverts to type.

And how easily he does so. It's instructive that he's more than ready to be convinced by Fanny in Ch.2. Especially at the beginning, their exchange shows that, far from having to be browbeaten into agreeing with his wife's arguments, he in fact enlarges on them on his own initiative. It's he who points out that if Harry were to father a numerous family the £3,000 would come in useful, he who without cavil gives form to Fanny's objection by proposing halving the amount he's offering, he who first proposes an annuity rather than an outright gift; and if sins of omission are taken into account, he who discusses, every bit as coldly and dispassionately as his wife, how many years Mrs D might be expected to live (note that he is only concerned with the amount of money he might be expected to forgo and the inconvenience to himself involved in annuities), and he who raises not the least objection to Fanny's unpleasant slighting of his own recently deceased father ("you owe no particular gratitude to him, nor attention to his wishes, for we very well know that if he could, he would have left almost everything in the world to them").

His loyalty is to himself - not to the rest of his family, nor even to Fanny, because she is simply an amplification of him ("Mrs. John Dashwood was a strong caricature of himself; more narrow-minded and selfish"). She does not change him for the better or even for the worse, in the sense that she does not encourage him to adopt any values different from his present ones. She simply magnifies the unpleasant qualities that are already inherent in him.