I suspect you're underestimating? A lot of people (around half of Americans if not more) don't carry a balance. I don't know what fraction of them try to extract the most from their cards, but judging from the sheer number of websites that explain what card you should get to maximize which reward and bonus, I'm skeptical that the number of people who pay attention to this is negligible enough that it'd cost companies more to simply close their accounts than to let them go on.