they are actually in the stadium or, more commonly these days, watching on their HDTV at home. TCU doesn't care a whole lot if their stadium only has 20000 people in it if they continue to win, are getting good ratings and raking in all that new tv money that every P5 conference now has.
you eliminate fans and CFB just becomes a ping pong contest...so in theory the games could still happen but no one would know or care.
while i'm not directly writing checks to tom holmoe or bronco mendenhall my spending money (tv, travel, tickets, merchandise, etc) fuels this ever growing, multi billion dollar business...of which BYU is a part. this dedication and sacrifice also is the reason we feel somewhat "invested" in the program and feel justified in griping when things continually go wrong.