Mail replaced segmentata? Why?

[SOCL]

I am under the impression that the segmentata wasn't an black-and-white, over night change, but rather it was a gradual change over time, hence the change in design from the BCE Kalkriese model to the 2nd Century CE Newstead. It also seems to me that there wasn't so much an overall change as it was, perhaps, only certain units in certain areas, dependent on the needs of the area of operations. In that sense, perhaps segementatas were never a majority over hamata, but this is just how I seem to look at this. I think there are more learned members who can provide better information.