I'm not saying Holmgren would be the ideal choice. Maybe he would, maybe it'd be better to keep Garrett - I honestly don't know. But I don't think his accomplishments should be dismissed; think of the two teams he led to the Super Bowl.
Green Bay has been a consistent playoff contender for so long than I think people look back at what Holmgren did and take it for granted. They forget how moribund the franchise was before him, and for how long that'd been the case. He led them to two Super Bowl appearances, and one win.
And the Seahawks...? Think about it. He took the Seahawks to the Super Bowl. Which they may have won if not for some very poor officiating. And he did it with Matt Hasselback at QB.
The man can coach, and teach, and call a good ballgame. I don't doubt that at all.