The economy poses two factors for Michigan--does population shrink thereby leaving less talent and can you continue to sell out the stadium which brings in tons of cash to fund the programs, pay salaries, etc.
Michigan and I will add Ohio are both rust belt states that have been having their issues for a long time. The population of Detroit has been shrinking for a long time. The economy in Michigan has sucked for a long time so this isn't a new problem for them. Through it all Michigan has remained one of the top programs in the country and the passion their fans have for Michigan football means they will continue to have 100,000+ fans at every game until they fall flat on their face for a while.
Sun Belt states have been growing at a much faster pace than midwestern states--albeit a lot of that is aging population so it has little impact on college athletics--and it is one reason SEC schools have been getting better and the Big Ten hasn't been doing as well.
I don't think this trend, or the economic problems in Michigan/Ohio, spell doom for Big Ten schools or Michigan in particular. What it does mean is that the proportion of top talent that is produced in Florida, Texas, etc has increased versus where it was 30 years ago and for a team like Michigan to be competitive they have to go out of state more for their talent. That is already happening with Dick Rod. Michigan has a brand that is seen as prestigious which attracts talent, the reputation for the quality of the education is second to only Northwestern in the conference and top 30 in the country which attracts athletes, and everybody wants to play in front of 100,000 fans every week.
My hunch is that they will battle right through the tough economic times.