Reasons for switching to a fully Free Software mobile OS:
Only then FairPhone respects our freedom as computer users
This would help making mass surveillance of entire populations uneconomical
Not only hardware but also software mainstream major corporations of the mobile phone world are heavily lobbying for disastrous trade deals like CETA, TTIP & TiSA, which threaten to destroy democracy, basic rights, and social standards.
In terms of voting with your wallet, acting against said disastrous trade deals & lobbyism regarding mobile hardware means buying a FairPhone, and regarding mobile software means using Replicant OS.