One benefit that Citi gets from the churners is cheap advertising. They know most people will probably tire of churning but keep the card. The people who churn also talk to others about the card. Those people may apply for one or possibly 2 "new" cards - but again, they will tire of the churning.

Hard core churners are probably a very small percentage of the Citi cardholders so churning is not a significant impact.

Just my opinion.