While I agree that it might have been nice to be able to observe Marianne's affection for Colonel Brandon unfolding, & I agree that chapter 50 does have a bit of a rushed feeling, I can't believe that anything JA wrote was anything but deliberate so she didn't just send off her outline in order to make the printer's schedule.

It does seem a bit offensive to think that Marianne should be the reward for all Colonel Brandon's goodness (& I don't think he would have accepted it that way nor would Marianne). I think that Marianne may have come round to view Willoughby as the appearance of what she wanted in a man but then learned that Colonel Brandon was the reality. Willoughby had the appearance of fulfilling all her demands in a romantic attachment; he agreed with all her opinions, seemed to share all her tastes, had lots of energy & was as cute as a button. She believed his physical charms were the outward manifestations of his internal virtues. That opinion was dashed by her knowlege of all his selfish behavior.

I think after her illness Marianne was ready to see the world with new eyes. I think she was able to see that Colonel Brandon was the possessor of all the virtues she had formerly attributed to Willoughby; she recognized his quiet, generous & unpossessive passion for her, and that awakened a true affection in her for him.