I have always thought this, and my mind boggles to 2009 and would have loved to have seen DC racing Button whilst challenging for the WDC, as that was Seb's first year in RB. I don't know if it would have been nearer or if DC would have done any better, but 2 British Drivers challenging for the WDC would have been fantastic!
Coulthard had obviously passed his peak by 2008. Him racing for RBR in 2009 would have been a waste for the team and he'd have been well behind Webber. Granted he scored a podium in his final season, but he was very fortunate with the safety car and the regulations about pitting at the time. His team-mate was actually in 2nd when the safety car came out, but Red Bull decided to pit him the lap the safety car came in for some strange reason. It was the race after the pit lane rules were altered and I suppose Red Bull simply forgot about this. But I digress.
The only good DC being at Red Bull in 2009 would have done would have been for Mark Webber. His stock values would have skyrocketed as Coulthard floundered whilst he challenged for wins.
DC no doubt did a lot of work to stabilise Red Bull into the team it is now, but a lot has changed since he retired. I know he stayed on as an advisor, but one must wonder how much an advisor actually
does. Schumacher stayed on at Ferrari for a bit and what was there to show for it? A few fans claiming that he
was the reason for all their bad strategy moves. He'll forever be remembered in Red Bull history, but the team has grown and matured much more than it ever did with him in action.