You're not really a "competitor" if you are using Google's sync backend, for one Google has no obligation to provide that to you for free, for two, if Google is

You're not really a "competitor" if you are using Google's sync backend, for one Google has no obligation to provide that to you for free, for two, if Google is storing and controlling the users data you aren't getting any value from it and the backend also won't be compatible with any feature changes you make.