Well, let's look at why tablets have replaced PCs for many users.
We can note factors such as reading comfort, mobile multimedia and taking notes without having to go to the desktop are superior, and we can not rule out the much more variety in design.
Between a PC and a tablet, the winner for the normal user is a tablet: it brings more obvious benefits.
Now, is the same true in the mobile phone vs tablet case?
The truth is that the functions of a mobile phone and a tablet are quite similar, discounting calls. Both provide mobility and basically the same applications. Anyone using Android for tablets automatically knows how to use the phone version, and vice versa. Not a drastic change
In this case the determining factor to choose is personal preference. Surely mobiles provide just what a large segment of the mobile market need, but there will always be those who require the extra size and performance a tablet brings, so I don't think tablets will be replaced completely.
I guess the best option for in-betweeners are phablets