You're thinking of the right problem but solving it the wrong way. Why do real life racing drivers not experience this problem? Because each season represents a different challenge and a different experience.
We all know that you can spend as much time in the one series as you like, but if every season has the same drivers, teams and pecking order, what's the point?
Infinite replayabilty would be possible if the game had it's own universe with drivers and teams coming and going from form, seriously different experiences from team to team and far greater variables in terms of contract possibilities. What if you finalled earned a spot on a leading team, only for them to switch makes and lose their star engineer to a different team, leaving you with a poor baseline setup and not much in the way of leadership. On the flipside to that, what if you punched above your weight during that difficult season and earned the recognition of the works team, who trial you as a co-driver in a few late-season endurance races before signing you for the new year?
And with DLC, things can remain fresh and exciting throughout the course of an extended career. For example, new GT3 cars will surely be added - would you want to be part of a team making a switch to the latest machinery, or would you prefer to keep your distance from an unproven product?
There is so much potential for a career mode with so much depth and replayabilty and most of it is already there. It pains me when I see talk of menu music, trees and customizable race suits, because in the grand scheme of things, isn't this so much more important?