There's evidence that the command track is a specialized career path within Starfleet, one you have to choose to pursue in order to become eligible for a command post.
There's also evidence, in terms of Dr. McCoy and the medical field anyway, you can still be a command officer, but not have been in command of any starship. So I don't see why La Forge couldn't have been a Captain of Engineering
, and not been forced into the command chair.
But I recall that LaForge's demotion in rank was mentioned in Plagues Of Night
as being a personal choice instead of being demoted for punishment, since I remember that he mentioned that Starfleet wanted to give him command of a ship because he was at Captain rank and Starfleet felt that, with its recent losses to the Borg, they shouldn't have anymore than one Captain aboard a ship, and that Geordi would be of better use on another ship.