You could be right. But Stonebraker is not "just another guy" predicting the end of the era of the RDBMS. He's one of the guys who invented the RDBMS. If he thinks he's seen something better, well, I think it's worth at least taking a look.
I've no comment either way on whether it is, and I know who Stonebraker is and respect what he's done, but he has a vested interest here. Just like Netscape programmers needed to kill off their earlier work, Mosaic.
I'm not going to form an opinion on his word alone, but he's got big enough chops in my book to warrant taking what he says seriously, even if he has a vested interest.