I don't think open source can get a big market share but it can give you a nice niche market of tech enthusiasts if they play their hand well.
For that they need to not let the development of the OS to just the graphene os team, and to have competitive hardware, software and prices.
By Motorola partnering with Graphene it will allow them to get a bigger market share and also help create a niche market for open source… It’s a win win