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Why was it impossible for Henry to make it to the end (even though I do believe that he loved Fanny) when other characters in other novels do?
Love does not change a person's basic character. Elizabeth makes this clear at the end of P&P when she tells Wickham Darcy, while improved in manners, hasn't changed "in essentials." Henry hadn't developed the self-discipline or the perseverence to stay the course. It's not a romantic ending but it's deeply realistic and true.
I believe JA's original ending is important, even though I succumbed to the temptation of that tantalizing "what-if" myself when I wrote Everingham!