Exactly, that's something people are not taking into consideration. Let's remember the game will be launched with the 2018 F2 season. If they let you race a full F2 season they would have to consider the possibility of Russel never winning the championship, but making to F1 anyway. That would be weird.
Problem I see with full F2 season in career mode is it might "break" the storyline. People say they want to play a full F2 season before going to F1. How can you be so sure that you will make to F1? Let's say you finish 10th in F2, how the game is gonna deal with it? Are you going to F1 no matter what your results were in F2? Then it doesn't make sense having a full season. On the other hand, will it be possible to not be chosen to F1 after a full F2 season? A lot of people would be disappointed if that's the case.