There is no question that there aren't enough screens on airplanes. You may be lucky to land a seat with a screen on a flight within the states. Internationally they become more common flying with companies like Emirates and Qatar airways. I am here to tell you, the number of screens you're seeing will not increase, it will likely decrease.
If you're like me, you use your phone before the flight until the last second that the plane is taking off into the air while you're still connected to your carrier. That is the desperation of connection with your personal device. The biggest trouble of being on a plane is that you aren't connected to your phone through your 4G or Wi-Fi.
Most inflight entertainment comes from handheld devices; smartphones and laptops, which is no mystery to the airlines. Inflight entertainment provided by the airlines is seldom utilized unless its free, but what the passengers are willing to pay for is Wi-Fi. In the world of technology, Wi-Fi is the key to life. Would you rather turn your plane on airplane mode and sit there for X hours twiddling your thumbs, or connect to the inflight Wi-Fi for $7.99 and reengage with the world. Many passengers would and do go with option 2.
Your handheld device offers several options and services that the airlines would never be able to offer for a reasonable price, so "if you can't beat them, join them". Airlines will turn many of their resources to providing entertainment options through your own device.
For airlines being able to connect with you through your iPhone it not only allows them to provide a much greater array of data, it completely cuts out the maintenance cost of supplying the electronics.
Planes are so difficult and arduous to build that by the time they come out with new ones, they are already outdated. Providing world class Wi-Fi keeps their customers with the best possible technology, their own.