Everyone uses Wi-Fi. Everyone loves Wi-Fi. It stands to reason that we will have lots and lots of it in the coming decades. The Internet will replace phone service to a large degree. Already you can do everything you need a phone for online, including make phone calls. You can even make calls for free online, which is a major incentive for people to switch over.
Wifi hotspots will eventually go away. Our Wifi will be all secured - no open Wifi except maybe at hotels and stuff - but otherwise mobile data will be so prevalent that 100% of the land area of the world will be covered with 4G service, or probably 5G by the time it happens. Mobile data has much more range, so instead of tons of routers that can break, mobile data only uses a few towers to cover the same area. Transferring between a bunch of hotspots every few seconds uses more battery and would cause more interruptions than transferring between mobile towers every once in a while.
In the future, I firmly believe that Wi-Fi will be the way forward. Whether that be much more powerful transmitters, or new methods such as using cables to transmit. Also, in the future, I can see existing infrastructure such as lamp posts and benches being used as transmission points. I do still feel that mobile carriers will be needed, as many rural locations will never have enough transmission points.
Most wi-fi access is locked therefore it is unlikely that the need for mobile carriers will vanish. What we need are some honest mobile carriers that aren't so concentrated on profits that they screw over their customer base. After paying over $200 to get out of a T-Mobile contract I vowed to stay out of contracts forever.
Mobile carriers are here to stay but their services will change over time. Although Wi-Fi hotspots are increasing in frequency and quality, people require phone and data service when they are at home, on the road and in small towns. More access to Wi-Fi will change how smart-phones are used but it will not eliminate the need for mobile carriers.