What does the B-Spec Career bar do?

Is he forced to retire at full bar?

I know that I've watched a YouTuber say that at level 40 the career bar stops increasing and that at three quarters he's starting to have enough of it so his stats decrease, does that mean if my driver reaches level 40 before it fills up 3/4 or full his stats will remain intact? (I'm expecting no, but it doesn't hurt to ask.)
United States
Madison, WI
At level 40 your driver’s stats will be maxed out.

It’s also worth pointing out that the career bar is different from the experience bar, so a level 40 driver should be at their ‘peak’ if you will for a while before their career bar reaches a certain point and their stats decrease. You don’t have to get rid of your drivers but they will become tired.