If Tywin was still kind to his children and actually teach Cersei what it means to be queen and how to be smart things would be so much better for the Lannsiters. And while Robert would have been king, there was no way Tywin would allow him to marry Cersei knowing his reputation as an adulterer and would be an embarrassment to his family for humiliating his daughter like that if he still had love left for his daughter and would know Joanna would never want Cersei to be in a miserable marriage. It would never be worth it seeing what a crap king Robert was, especially as a husband.