From the animation on FairPhone site, I counted 17 screws and 6 standoff. How many people, who would not even try to repair their phone will find this easy enough to start repairing their phone?
For comparison sake, there are 10 screws on a Nexus 5 (according iFixit teardown). Sure, the Nexus 5 (or other smartphone) will be a little harder to repair. But the difference is far from huge, and won't make someone take a FairPhone over another "good enough" smartphone, repairability-wise.
The upgrade ability? Sure, it might be nice. But, as the PC market as demonstrated (which has a lot less contraints, too), you can only go so far, until you need to replace the whole thing. Since it's not a well-established project, it's not even a sure-bet you'll still find parts at an affordable price in a few years time. And at that price point, it's not a given that FairPhone + upgrades in a few years costs less than one similar phone now, and a similar to the upgraded in a few years.
That leaves social impact and openness principles. That might appeal to part of the geeky crowd (the ones that buys Librem), and social-aware persons, if they are not turned off by the price. Let's see if it's enough to sustain a smartphone line. In the current saturated market, I have some doubts.