Wolverine has been a huge lynch-pin in the X-Men franchise, and while you can argue they are a proven franchise, they've only ever proven themselves with their most recognizeable characters. There is nothing to suggest a movie or show can exist without some combination of characters like Cyclops, Wolverine, Storm, Gambit and or Jean Grey. bad place, they can barely do that in the comic books. Wolverine isn't in every book and making guest appearances 'cause it's a good way to stay active ya'know!
They'll have an uphill climb. First of all, people who don't follow every piece of movie news are gonna see the First Class trailer and be like "WTF!?! where are the X-Men. Azazel, Darwin, Havok? Who are these weirdos". Now that doesn't necessarily mean they won't be interested, but it will mean the movie will have to convince them it's worthwhile.
Side note: Aside from comic accuracy, marketability is another reason I think McAvoy needed to be bald in this movie. Professor X is iconically bald. It's something that people expect of that character. I kind of feel it'll be hard to think of him initially as a younger version of the Patrick Stewart character if his head isn't shaven.