Only the extreme heavy users are streaming over cellular though, other than maybe a short YouTube video, which is fine on a 3.5" phone.
Most people don't use YouTube or update applications on cellular. Applications can wait for Wifi. The user is question does email, maps, and occasional web.
The carriers aren't hurting anything, as even heavy use is well below 2GB/mo, with the exception of streaming radio, but there are ways to control that as well.