I agree that the film was maybe 15 minutes too long where the last bit could have been condensed into the preceding scene somehow:
I fail to see how the added 15 minutes is QT letting his own hype go to his head though, the prior 150 minutes were excellent, and the ending didn't let it down exactly, it just felt like it was stretched a bit where it could have been condensed. He's one of the most unique film makers working today, and if he keeps giving us films like Django i'd be very happy.
I think that scene was included to illustrate the horrors of slavery, and the things that slavers did to keep their chattel under control. If the film had ended where you suggest, (which I agree, it should have,) Django could still have freed all the slaves, and they could ALL have ridden off into the sunset together!