One of my best friends is a Vandal and is having a melt down. His dad, also a Vandal, is happy with the decision. As an objective observer you can make arguments for going or staying, but some of the arguments I'm hearing from the FBS crown aren't really all that true:
Attendance will drop. Many Vandal fans don't care about the Big Sky
Possibly. They may lose some attendance initially, especially if they don't win. However, I believe winning brings the crowds regardless of what level you play. Actually 6 of the 10 biggest crowds in Kibbie Dome history were before they moved up. 7 of those 10 were against Boise State, who has refused to play in Moscow even if they do stay at the FBS level. Only 1 of those games was against an FBS team not named Boise State and that team is New Mexico State, who may also drop down to the Big Sky. 2 of the 10 are against current Big Sky members. 0 were against Sun Belt members.
Dropping down will drop National Prominence
Nobody cares about Idaho football outside of the Northwest. Same can be said about the Montana schools. There are different indicators that you can reference "national exposure", but there are several indicators that show Idaho has lower national popularity than many schools with FCS programs:
Apparel/ Merchandise Sales
If you gauge prominence by national popularity and look at merchandise sales there are several colleges with FCS football programs that have outperformed Idaho. That list includes Montana, Montana State, Delaware, and James Madison. There may be more, but the list only showed the top 75. I won't include Georgetown, Villanova, and the Ivies.
Television and ESPN
Not counting ESPN 3 (because that really isn't national television coverage in my opinion), Montana outperformed Idaho last year. Idaho and Montana had several games broadcast on ESPN 3, but only Montana had a nationally televised game. NDSU had at least 2. Idaho fans may argue that their Auburn and USC games were televised to a large audience, but they got housed in those games. Not exactly the publicity you want. In contrast the UM vs NDSU game was televised in Missoula and the result created a lot of positive buzz to the program.
Quality of recruits will decline with the drop
I think even with Idaho being in the higher division, they haven't been able to sign better quality players than a lot of the Big Sky teams. In fact, I think Montana has beat out Idaho on a lot of recruits. Why? Because players want to win. Losing isn't fun and for many demoralizing.
It will jeopardize the prestige of the University
Doubt it. There are many FCS schools that are very prestigious academically. Some of those schools such as Davis and Poly are very highly ranked. Montana, UND, ISU, and Montana State are also strong academic institutions that are peers to Idaho.
Just a couple arguments from me. There may be other logical pro-FBS arguments out there, such as cutting other sports programs and layoffs that might be a negative. However, just because Idaho is dropping down, does not mean they need to drop any sports.
I'm under the opinion that if a program can't average at least 15k home attendance and does not have the resources or booster support to invest in the facilities and financial needs of a successful program, that program does not belong in the FBS. There are a lot of schools besides Idaho in the FBS that don't belong there and that are irrelevant to the vast majority of football fans.