I will disagree (I know you and I have a bit on this subject). I wouldn't have even looked at WSU if it weren't a P5 school. I'd imagine there's plenty like me still out there in youth-ville (and I really don't enjoy the college athletics landscape as much any more). If/when this happens, what does WSU offer that is exciting? G5 sports aren't going to be a selling point. The "No. 2 public school in the state" isn't going to hit for the major academics, who likely go out of state, private, or to the state's No. 1 public university. It's fun? Sure. I had a great time - and the weekends around alumni were super fun who came for football, but those numbers will likely dwindle to a MWC/PAC hybrid.
I was on campus for the "game day experience" with Paul Wulff years. Pullman during those weekends were a major bust (I envied those kids during the Leach run years). Although it wouldn't drop to that level, there's likely a shift in that experience, too.
There absolutely should be no Apple Cup, assuming UW bolts elsewhere. WSU doesn't owe UW anything (as UW owes WSU nothing back). Even when Utah and Colorado joined, those were "meh" type of game weekends. It was about the Oregon, U$C, UCLA (and Oregon State, regional purposes) types of games people got excited for.
WSU will have to really maneuver their marketing of the university as a whole if/when that time comes.