McNabb was certainly better than a lot of his backups, but his backups did alright as game managers...but that was all they did. They managed the clock, the Philly offense was much better with McNabb than it was with his backups. The Hummer scored a lot more points and won a lot more games than the Honda, even though the Honda still managed to run.
My jury would be out on the Kolb thing. Reportedly he has a much better rapport with Philly's receivers than McNabb, so there might not be quite the dropoff that you think there will be. Additionally, we don't know if Bradford can run the WCO effectively, whilst McCoy had Texas' offense clicking nearly as well as Oklahoma's spread and has less of a learning curve anyway.
In addition, this isn't the same as cars, a car just needs to go from point A to point B and as long as it runs it does that. In the NFL, it isn't as simple. A QB who can run a WCO doesn't mean he runs it well or is as effective as another more talented QB. McNabb and Philly proved that by losing a large chunk of their offense with McNabb even though they had a QB that still could run it.