Perhaps it can't for you...my brother had a surface pro and just recently got the 6+...says he never uses the surface anymore for anything, work or otherwise. Obviously this is a case by case situation, but given the abilities of the iPhone and iPad being almost identical, I think it could easily replace a tablet for some people.
Not sure what he does for a living. But the surface is basically a complete windows Ultrabook hybrid. It can do anything a desktop can do, so it's pretty much what I said. There's a metric ton of stuff this phone can't do that a tablet like a surface can do, like run full windows software like Photoshop, Corel Painter, Dartfish, or even a Visual Studio IDE.
Smartphones are very capable so I'm well aware they can "replace" a tablet for users with quite basic needs. However, the only tablets this phone can truly replace are those running fairly stock smartphone operating systems, like an iPad Mini or Nexus 7.
Yes it's definitely case by case as to how it fits for the person, but it's not something that can generally be called a tablet replacement by anyone who uses a tablet as anything more than an oversized smartphone.
Many people are using it in lieu of a tablet because that's all the tablets ever were to them.
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