I find this kind of amusing. I suspect the adoption of this varies by area/country based on how, ahem, well, poorly basic SMS is implemented and priced in that area.

For example I notice that when in Germany, Whatsapp adoption is quite high, because SMS rates are absolutely ridiculous. Other countries... less so.

I still get SMS (and email, but SMS frankly still pushes more reliably than data based systems) notifications for my airlines, and that also certainly qualifies as Not An Extra App :)