‘Sansa was kept alive because she was useful to Joffrey’
No. Sansa used her wits in order to survive, and her ‘titles’ meant squat when the mob were rioting, or Joffrey tormented her. Her titles did not protect her from being molested by Tyrion, being forced into a marriage or having Cersei want to kill her.
Stop with this BS that Sansa’s survival is because she was ‘too important.’ It may have been why Cersei didn’t harm her (although she did psychologically abuse Sansa), but Joffrey didn’t give a shit about Sansa’s ‘importance.’