WiFi uses microwave frequencies. These frequencies are not valid for long range communications because they are absorbed by air humidity and walls.

If mobile data usage does not increase very much, most public WiFi hotspots will disappear. But if people start consuming more and more mobile data by watching videos or using cloud storage, mobile networks will get saturated again so unlimited data plans will be more expensive or even disappear and WiFi use will remain. Unfortunately I find more likely the second case.