I'll probably get flamed pretty bad for this, but here goes...
City driver's don't make great road drivers.
Road drivers don't make great city drivers.
This is evidenced in the companies that have used single boards.
Over the years, (some drivers) can develop their craft to the point of an art form.
I've had the pleasure of watching 30 year veteran road drivers go down the road, making a FedEx set look like a long nose chromed out Pete. They are never in a situation, never in anyone's way and never flailing. It's a beautiful thing to see and the nuance and subtlety would be lost on many, who don't understand what they're looking at.
The same could be said for a veteran city driver, juggling navigation, customer relations, bumping difficult docks
etc...
An NFLer doesn't switch positions for good reason. We can all get specific skills that can only be learned with years of repetition.
Besides, with autonomous trucks in the future, city drivers should have more longevity anyway...