Been following this for awhile now.
Likely to be (at least at first) a very niche device: most andoid users are well steeped in modern consumerism, and are comfortable with buying the new "latest model" every 2 years. There does seem to be a fairly big niche for this, though
One thing that is going to have a big effect on the Ara's success or failure is going to hinge on hardware manufacturers. If the different hardware companies get on board, the Ara will be a success. If they only have Motorola making the parts, it'll fail (or just limp along in the niche).
As for swapping out the screen: my understanding is that the screen is also supposed to be swapable (including being able to switch from a large touch screen to a smaller screen with hardware keyboard). Not sure if that's how the final product will go or not. I know an easy to replace screen would, all by itself, eliminate a large number of phone sales...