I read this interesting research from Good Technology (I dont work for Good or have any association with them), but found their tracking study interesting...
Summarised here.
http://www.kumulos.com/2015...
In short it shows a shift towards Android and Windows at the expense of iOS for recent tablet sales. My guess is that this is being driven (in a large part) by big adoption of tablet devices for business apps (workforce efficiency/mobility), when you have the app running on hundreds of devices and iPad is 4x the cost of a decent android tablet.. and you can control the form factor & OS versions it makes real sense to go Android....
Seems to say that apple will own the consumer market but Android (and increasingly Windows) will dominate the (arguably much more attractive) corporate market for workforce apps.