Did you have a smartphone 10 years ago? Do you remember what 3G was like? Wasn't so bad, right? Try using 3G in any metropolitan area today, a lot of times it's painfully slow. Why is that? Congestion. The reason to upgrade isn't necessarily to improve the best case performance for a single user it's to provide a better quality of service for a larger number of users. Anyone who's attended an event like comic-con or PAX knows just how easy it is for even 4G LTE to get saturated with connections.
And cell carriers running 3G networks with minimal spectrum. Why leave spectrum on older protocols like 3G, when it could be making your LTE network faster? Hence T-Mobile killing 3G in many areas, with 2G stuffed into the guard bands of their LTE broadcasts. Not much spectrum allocated for 2G either...
