A Third Of New Cellular Customers Last Quarter Were Cars (recode.net)
Ina Fried, reporting for Recode: With the U.S. smartphone market saturated, most of the growth in the cellular industry is actually coming from other kinds of devices including tablets, machine-to-machine connections and lots and lots of cars. In the first quarter, for example, the major carriers actually added more connected cars (Editor's note: amounting to a 32 percent capture) as new accounts than they did phones.
So the car can send telemetry data back to the manufacturers to let them know what you're doing to the car so they can reject warranty claims. (See the story about the Tesla that auto-drove into a truck.)
I mean, to enable exciting new features like live updated maps and traffic in the in-car navigation system and things like in-car wifi! Totally not to constantly spy on you and everything you do!
For my car, a 2016 Camaro SS, there's little that will be broken. The only things you would lose are the ability to make/take call on the car's phone number, you can still pair your BT phone just fine as with most modern cars. If you bought the Nav system, that is independent and will function just fine, but you don't get Directions and Connections from OnStar anymore.
Your XM channels and traffic are a fully separate thing you can choose to continue or not.
Honestly outside of the remote diagnostics and the OnStar App functions, you don't lose much. (for me, anways).