Every team is subject to the possibility of significant injuries, and it could also happen to the Cubs, although career ending injuries seem somewhat less likely to young pitchers than to young hitters.
In addition, the Cubs are slowly building a minor league system that can provide adequate replacements to the injuries that are almost sure to happen, just as the Cardinals have done over the years. They are probably still about a year away from bringing that to fruition, but it is coming, if they stay the course.
My goal for them is to have them become a 10 to 15 year dynasty, and I think that they are nearing the point where that is not an unrealistic hope.