Going a whole season without a trophy is definitely reason enough for Madrid to sack their coach. There are already talks about his sack amongst the fanbase. One of the factors that can determine his sack is if he doesn't win Barcelona in any of the two remaining classicos of the reason. It may sound silly but they take that rivalry seriously. And to be honest, Madrid are not really playing very well. Against Barcelona they couldn't get a single shot on target at home, that's a very bad stat for a team like Madrid. Ancelotti"s job is on the line and he knows it. It is not a certainty that he will be sacked if he doesn't win a trophy this season but its a possibility.
With his current appearance, it seems that Madrid have very little chance this season to bring home the title. In La Liga I think the title already belongs to Barcelona, then in the Copa Del Rey there is still a 50:50 chance for both of them. A team's setback is indeed the full responsibility of a coach, but in my opinion it is very unfair to have to fire Don Carlo in the near future. Problems in the Los Blancos squad were detected towards the end of the year and added to by Benzema's injury at that time. Madrid's management should have spent on players in January transfers to add strength and keep the team's performance stable.