As far as Firefly is concerned, that will always be unfinished business. Serenity was a Band-Aid on a sucking flesh wound. I think every day about the scenes that I’ll never get to shoot and how badass they were. It’s nice to know that people still care about Firefly but it’s actual grief that I feel. It’s not something you get over, it’s just something you learn to live with.
While I understand Whedon's grief, I wouldn't let it prevent me from working on a sequel if a studio put up the money. Yes, there will always be an unfinished feeling to Firefly, but as dramatic as Joss's quote sounds, he has worked on Serenity comic books since the movie. If he thinks about unfilmed scenes as often as he says, you'd think he'd jump at the chance to film some of them.
As much as I'd like to see a sequel, I have to accept the possibility that Joss's heart isn't in it.