It is not impossible, and whilst I think Souths will more likely than not finish in the top 4, I have my doubts about them going back to back. Losing Sam Burgess is a big blow for them, he is the cornerstone of that pack on work rate alone. Whilst Tim Grant and Glenn Stewart are good players, they are not in the same class as Sam Burgess work rate wise.
Then toss in having probably more players than they have hade involved in the Four Nations this year, the earlier start to the Auckland Nines in 2015, plus having to travel to the UK for the World Club Challenge, this is going to put a heavier tax on their player's both physically and mentally.
I am not saying they will be unfit, but all that extra stuff cuts down on recovery time as the rep players won't get much of an off season break -or it will cut down on their pre-season. Pre-season work is where much of the foundation is laid and Maguire (like Trent Robinson found this year) will find he won't have as much time to work with those players before they are back playing games.
Then add in the emotional "comedown" from winning the comp this year. Some may find it harder in 2015 to put in that extra effort that they did in 2014.