I think that is it - it is a contrast of how the Musgroves can still love a son (or the memory of a son) who the ON doesn't think was worth loving, yet Sir Walter only loves what is in front of him and is pleasing to his eyes and ego. I don't even think he really loves anyone but himself and Elizabeth. But if he really loved her, you make a good point that he wouldn't put her in the position of a precarious future.