Very possible, not "almost certain." First of all, as you wrote, Leach has brought consistency to the program. That means his replacement will be coming into a more stable situation than anyone else has come into before in Pullman. Secondly, assuming we pay the new coach something close to what Leach is making, that's a lot more than what other new coaches have made coming to WSU. That should buy something better than going back to 3 win seasons. Yes, that assumes the AD makes a good hire, but I have faith in Chun to do just that based on what he has done with other WSU athletic programs.
The most desirable scenario for most of us is that Leach stays. But despite our long history of overall ineptitude in football, we are in a much better place right now to sustain success even without Mike Leach. I'm not afraid of a future without Leach.
Glad Cougar
On this, I think your overall point is pretty reasonable in that we don't currently have some of the disadvantages we had historically. The stadium has been improved, we're paying a market wage to coaches, etc. We no longer have to try to hire some Big Sky coach for half of the market wage.
I just hope people understand that paying a market wage and having median-level (or below) facilities in the conference doesn't guarantee success. It is necessary, not sufficient.
Similarly, having a baseline level of lower-tier, arguably solid Pac-12 talent (which is what we have with Leach) doesn't guarantee a subsequent coach can win at a high level with it. Objectively, by the numbers and in the aggregate, the talent we have coming in the door each year is in the bottom quartile of the conference. Those numbers aren't completely reliable, but the current staff's ability to win with lower-ranked talent is as good as it gets nationally and is unusual.
As far as another coach's ability to recruit, keep staff, etc., we have all the disadvantages I mentioned earlier (recruiting territory, budget, history, prestige, etc.). Even if our other "stuff" is as good as everyone else, history and looking at other programs around the country, currently and historically, bears out that this doesn't lend itself to ongoing success. The only "have nots" that have become sustainable winners have had massive infusions of cash into the program (e.g., Pickens, Knight) and/or have had a HOF-level coach that has stayed forever (Snyder, Beamer), and the latter case is questionable in terms of sustainability in and of itself ... at those schools, it has led to significant increases in donations and facilities. Even today, I can assure you nobody in Big 12 country thinks of Kansas State as a "have" or a sustainable winner, for instance.
In the case of Leach, I can recall sentiments among Texas Tech fans that were similar to some of what we've heard from WSU fans recently ... I was looking around the boards when we hired him and I saw some mixed opinions similar to what we're seeing now around the time he was fired. Some were tired of him supposedly shopping around for other offers, etc., while most loved the guy. Tech had expanded the stadium multiple times, had attained a degree of national relevance, etc., but a segment still didn't like the guy and was sure they'd be fine, even better off, perhaps, with Tuberville. Tuberville was hired from the SEC and paid a good amount of money, and many of the arguments (as I recall) were exactly like what we're seeing from some fans here ... everything would be fine because Tech now was a sustainable winner, Leach had improved the program but couldn't get it over the hump, Tuberville cared about defense and wanted a more balanced offense, etc.
Well, Tuberville failed miserably. Then with Kingsbury, everyone thought he was going to be like Leach but even better, with a more balanced offense and various attributes that would allow national attention, great recruiting, etc. Well, he failed, too ... he had a decent first year with Tuberville's / Leach's players and was viewed as doing OK, then just flat-out couldn't get it done.
Are we doomed to the exact same fate as Tech? Not necessarily, but every single piece of objective evidence, and a two-part example that is as on point as you could ever hope for, suggests that we're in for something similar if Leach leaves, if not worse.