AT&T’s top executive says the era of big subsidies for devices is coming to an end, as wireless operators can no longer afford to fund a constant smartphone upgrade cycle. Speaking at an investor conference in New York City on Tuesday, AT&T CEO Randall Stephenson said that with smartphone penetration at over 75 percent and soon reaching 90 percent, wireless operators need to work harder to get customers to use more of the network rather than simply getting on the network. “When you’re growing the business initially, you have to do aggressive device subsidies to get people on the network,” he said. “But as you approach 90 percent penetration, you move into maintenance mode. That means more device upgrades. And the model has to change. You can’t afford to subsidize devices like that.”