I think there is a minority if the population who churn through the credit card sign up bonuses and the companies probably lose money off of them. But I suspect the vast majority generate miles which the airline will get paid for by the credit card company which may not be used plus there is probably some loyalty bought where people are willing to pay a small premium for the early boarding, free checked bag, etc.

I wouldn't be surprised to see more of the mandatory minimum spends for status and a further expansion of availability reserved for people who actually fly frequently as opposed to people who just have the cards.