"Generally" refers to an average trend, not that all examples must follow that trend.

I am not that familiar with Marfan's or this case, so I don't know if this device is useful in this case (http://www.medtronic.com/patients/heart-valve-disease/about-...), but there are stent-grafts that also replace the aortic value so that the stent-graft can be used close to the heart, and you can be sure that medical device companies are looking at ways of using stent-grafts in close proximity to the aortic valve without requiring its replacement.