Given Google's penchant for abruptly canceling platforms, I hope that implementors and vendors will be careful with this.
I'd be surprised if any vendor adopts Fuchsia thinking it has anything better than a 10-20% chance of being mainstream.
Today we have Android and iOS as the dominant Mobile phone OSs. Don't forget about all the attempts to dethrone those two by: Symbian, WebOS, Windows phone, Blackberry, Tizen, Ubuntu Touch, kaios, firefox os, PureOS, LiteOS,...etc.