OC to head coach is such a different skill set (CEO). Sometimes it can be worth it to overpay a brilliant OC (or DC) to let them lead your org, and there are many successes out there like that, but none really get to the very top of the sport without changing and evolving. Lincoln Riley, Leach, Venables, Tedford, all excellent on one side of the ball, but make them lead a team and they're able to make that team good but never great. I would even toss Sark in here though maybe he's been HC for so long now that he's learned that part (lol j/k).
No idea if Grubb has that in him - he might, but hasn't had the CEO opportunity, and if we've learned anything it's that you don't try that for the first time at Washington. The Kirby Smarts and Bob Stoops of the world are rare.