I believe it can be fixed for much less. A new landside terminal is not cheap. Furthermore, the land is not flat so there's some grading that will need to be done to bring the traffic up to the X structure.
I think they should just fix the roof if it is leaking, close off one or two of the concourses, leaving 2 left and consider whether it's cheaper to keep the train or convert it to a walkway with moving sidewalks.