We know that in all the X-men movies (Besides X-men First Class) Wolverine is the lead character. The whole story comes around him and unfolds with him in basically every scene.
We all know of a huge reason that he has the coolest badass fighting scenes.
Another reason would be that he brings the sorrow and ruin and death into the movies. His romantic life never works out because everyone he loves dies.
He's that one guy who really isn't a team player. The Movies are about Wolverine the plot always having him have the huge role. In [X-Men: Days Of Future Past](movie:203942) he had a fantastic lead role.
They also put him in X-Men: First Class for a short scene. Which that can tell you they need wolverine.
Hugh Jackman plays the animal..and he does a terrific job portraying him. Most people probably know who Hugh is because of wolverine. Hugh Jackman can't stay young forever. So if he retires and they get a new wolverine actor. It just won't be the same because wolverine is the main character. That's why if Hugh quits. I feel like the money that X-men gets will shrink. I've heard people say I only watch X-men because of Wolverine. That is why without Hugh Jackman who portrays Wolverine fantastically X-men could very well meet it's own demise.