Considering it's enabled by default, i'm really curious why you would want to do that.
Also if I switch to Android I don’t have to worry about disabling iMessage before. (Not that anybody in my country would ever send an iMessage or an SMS though...)
Not sure what pop-up you're talking about then.
This shows up when you insert a sim that’s not activated with iMessage. Maybe it depends on your region. In my country you really have to pay because the activation process sends an sms message.