I also prefer the split between the two, but I do think there should be a option for those who want to have a created driver in their lineup but have it as entirely optional. I don't think it should be owner/driver but owner/son-driver makes more sense from a believability viewpoint.
I guess they'd think that only being able to use a created driver in Driver Career would mean that mode would be used more. Kinda feels like they intended one to be a Driver Championship-centric mode and the other to be a Constructor Championship-centric mode but I'm not sure it really comes across that way.
If i'm honest, the moment they fix the two main issues with MyTeam (being the sub-standard customisation and incredibly shallow driver pool) then I don't think Driver Career will be all that popular a mode in comparison.