I don't necessarily agree with that. All of the conferences with automatic NCAA Football playoff bids, including the MWC, are going to see an uptick in their revenue.
A program like WSU would enter the new Pac-whatever or revised MWC with some advantages that 85% of the other member programs don't have. Better facilities, scheduling benefits, "alimony" paid by UW for leaving, etc. Top coaches in the Mountain West Conference are paid around $1.5M/year. WSU will be able to top that.
Comparing the drop Idaho made when they went back to the FCS isn't accurate at all. The top MWC schools routinely field better programs than middling P5 schools, and not only do I expect that to continue, I feel it's going to increase. The kings of the MWC, whoever it is that emerges, are going to have the opportunity to be the Gonzaga's of West coast football. Routinely playing for bids to the NCAA tournament. Fresno State is a better program than Cal. I'd argue that SDSU is a better football program than UCLA and Arizona. Boise State has been better than 1/2 the P12 schools for 15+ years now. Same with BYU. How much worse has Colorado State been than Colorado?
The revenue loss will be an initial hit, but then it'll level off. As has been the case for WSU historically, we won't be able to compete on coaches salaries with Super Conference programs; UW, Oregon, UCLA, USC, etc., but we'll still be one of the top jobs on the West coast.
A move to the only West Coast conference with an automatic bid to the NCAA tournament isn't the Big Sky.
Much of this is reasonable, but the revenue loss is more than an initial hit. It would be something like $20m less each year, even if we charitably assume WSU would get something like $12m a year from what you describe (e.g., $8m a year for an improved media rights deal, and $4m a year in alimony from UW), neither of which is certain at all. Neither even is likely, in my view. Cal is going to get between $2m to $10m a year from UCLA, with the initial range recommended at $2m-$5m, depending on how the Pac-12 deal shakes out. They're both part of the same UC system with the same regents. No guarantees at all that WSU would get anything, let alone anything material, from UW. There probably would be something but it wouldn't be all that much.
Regardless of what number you want to use, the link you shared earlier notes that the WSU AD still has $75m in debt, even with $36m refinanced, and hasn't formulated a plan to pay it back yet. It has a slight deficit projected for this coming year, even with the current Pac-12 distributions. So there's a debt overhang that will always be there. Combine reduced revenues with still needing to pay all that money back, and it's pretty grim, especially in terms of any kind of new facilities or maintaining the current ones.
Being in some kind of revised Pac-12 / MWC hybrid would let WSU spend less on coaches and it could cut some off-field staff and other things, but not by much. The department already runs pretty leanly compared to other P5 schools.
For a short period, WSU would likely do OK, but not incredibly well, in the conference. As you noted, the MWC programs aren't pushovers. Over time, it likely would settle in as a mid-tier kind of program in that conference. It would have pretty good facilities, but the better MWC teams all have pretty good facilities, too. WSU would be positioned decently to recruit against many MWC schools, but even places like Albuquerque, Reno, and others would be more appealing to most recruits, let alone Vegas, San Diego, San Jose, Honolulu (for some), and Boise. There's no escaping how remote Pullman is. The only real things WSU would have going for it vis-a-vis every school other than Wyoming in the longer term are being in a relatively sizeable state with a relatively high amount of wealth -- although most of it isn't held by WSU alumni -- and whatever mileage it could get out of having a history of competition in P5, although even that isn't all that remarkable. Recruits aren't going to care about WSU having been in the Rose Bowl 20-30 years ago.
All that said, I agree it wouldn't be as terrible as Idaho going to the Big Sky. WSU would continue to field athletics teams that would be competitive, if not all that good, in the conference, and if everything went perfectly, they could be pretty good for a year or two before the coach was poached, but on the level of, e.g., what Fresno State did last year, going 10-4 and winning a low-tier bowl game nobody cares about. In a really, really good year, if the winner of the conference was among the highest-rated 6 conference champs, the champ would get in the playoff as the 12th seed and ge the privilege of losing by 35 to Georgia, Alabama, or whomever. And again, that's best case.