Fitzgibbon has barely played more good games than can be counted on a single hand. The guy has tonnes to prove long term yet. There's promise there for sure, but there's a ways to go before he's an automatic start over any of our other options, including Sione. He may not even make the regular 17 next year.
Barnett has done enough to say he'll be in there somewhere, but I wouldn't say is guaranteed much more of a start than Sione. Barnett might have got player of the year, but Sione was named captain for a period, too. The coach obviously thinks highly of both. More than likely we're approaching a point where form will play a significant part in who is picked week to week, and that's a good place to be.
I'd be surprised if anyone in the squad is a 100% lock to start at this point, especially in the forwards. Ponga and Watson pretty likely to get first shot at 1 and 6 provided they have good off seasons, although Lamb and Hodkinson could certainly change that equation too.