Geppy Parziale, an expert on fingerprints:
If you search for the specifications of a CMOS fingerprint device, you will find a number representing the lifetime of a device. That number is expressed in number of touches (before it completely dies). That number is provided in ideal conditions of usage and in a normal operating environment of temperature and humidity. But remember where you normally use your iPhone. You keep it in your “dirty” pockets, you leave it on different surfaces, and in humid and hot or cold and dry environments. Sometimes water drops on it or you forget it in your car under the sun. All these factors stress the working conditions of the sensor surface and contribute to speeding up its decay process.
Unfortunately there is no existing solution to this. Manufacturers can only try to make the fingerprint sensor last longer, but sooner or later that device will stop working properly. This is also why Apple cannot provide a fingerprint sensor for payments. And if they do, they are making a huge mistake, because the surface destruction process explained above introduces the most dangerous problem in fingerprint recognition: false acceptance, when after a while somebody else can be granted access to your device.
While the rumors are growing in volume, I just can’t see Apple putting a fingerprint scanner in the next iPhone. What would be the point? A little more secure, sure, but what can you do with it? It sound expensive and fragile and worthwhile for only a fraction of Apple’s customers.