Also one of the drivers for Google is to prevent mobile from becoming a walled garden that is dominated and controlled by one company (Apple), which could easil

Also one of the drivers for Google is to prevent mobile from becoming a walled garden that is dominated and controlled by one company (Apple), which could easily decide at some point to replace Google (and there's already some talk that Siri will be the first step in that direction). Whether or not it's a major contributor to Google's profitability at this point in time, is really not so important as creating an independent mobile ecosystem where Google products can thrive.