Well I haven't seen that movie, so I don't know if it's so good - it looked cheesy to me, like so many Hollywood romantic comedies, so I couldn't be bothered.
Really the genre has gone downhill since His Girl Friday, from 1940. And the female lead in that movie was originally written for a man - that should give you a hint why it was a successful romantic comedy - the woman wasn't a complete loser.
One of the commenters on the Dobbins story actually suggested that "Knocked Up" and "Forgetting Sarah Marshall" were "successful romantic comedies." That should be a big clue as to why there's a problem with the genre - because that kind of "romantic comedy" celebrates misogyny, and pairs ugly, slobby man-babies with hot but uptight women. If "How to Lose a Guy" seems good in comparison to the "romantic comedies" of Apatow and the like, maybe it's because at least Matthew McConaughey is an attractive man.
Why is the romantic comedy dying? Because it's a genre run by people who hate its target audience.