Here's something to chew on:

The iPad uses what's called a microSIM card. Basically an readaptation of an external modem. This makes it logical to take your SIM out of your iPad and drop it into an unlocked mobile when you need more portability.

Both devices can connect to several online services to keep information synced.

Or, the SIM card becomes the persons hardware, software, and services validation key. Making the connection to a carrier not so much vital as much as it does point as an easier way to see how specific users are utilizing services - go back and look at Nokia's 2015 video.

This (connection to the web as a utility) is pre-paid. With such a decision on the hardware side, one would have to assume that usage based polices from carriers aren't that far away.

Just a little something to think about.