It didn't help that telecoms used to use SMS as an extreme profit center. I don't think WhatsApp would have taken over the way it did if SMS was always included in all plans for free. This is similar to the way "local" long distance used to be such a racket.
Most UK plans included unlimited SMS for a long time, but whatsapp still took over.
The group chat functions don't really exist in SMS (maybe in MMS but they never work properly), photos (same), whatsapp desktop, you can text when you have WiFi but no 4G (or using a different sim card when travelling), etc.
