The teams they are playing, except for a couple of bodybag games a year, are all teams like they are. Nobody cares about them, nobody watches them. It isn't getting them publicity or attention at any higher level than if they were FCS.
I don't know what Idaho's ticket prices are. I do know that the average non-student ticket to a CSU home game sells for a little less than half the average non-student ticket for a game in Boulder. They aren't exactly raking in the bucks because they are supposively a FBS team. Even at these prices CSU averages less than 1/2 the home attendance that CU does. Some simple logic means that they are generating about 1/4 the ticket revenue per game that CU does. Hardly a big time revenue producer.
You talk about massive jolts in attendance for incoming teams. Are you talking about the massive jolt CSU got when they had a top 10 Cal team come in and still had under 30,000 people show up. And that was when CSU was still going to bowl games.
Certainly they will lose some fans when they make the drop in divisions. Some of those who hold the fantasy that they are a big-time school will get mad and drop off. If they lose 25% that means going from 20,000 which is at the bottom of FBS to 15,000 which is very competitive at the FCS level. The cost of running the program will go significantly down and they will still have a budget that lets them be competitive. Get back to winning games and having some excitement in the stadium and they will gain fans back. D2 CSU-Pueblo actually had more fans in attendance their last couple games than CSU-Ft. Collins did last year because they were winning, it was fun to be there.
Unless they want to continue to bleed money as they are getting killed every year on the field schools like CSU and Idaho aren't going to have a choice but to look at where they play. Neither one is getting an invitation to a major conference. A new stadium won't change that, a magical big-money donor won't change that, none of their other dreams will change that. $1.5-$2.5 million a year in conference media revenue isn't going to make it when other schools are getting more than that per game.
You can't say forever because nobody knows what the long-term holds but short of a complete change in the college football environment schools like CSU and Idaho are deluding themselves if they think that they can play with the big boys. They are wasting their time and money trying to perpetuate a fantasy that has long passed.