I'm not really sure what Apple would have to gain by doing a pure fitness band. Fitbit, for instance, specializes in fitness bands and watches, which is why you can see in this comparison of all Fitbit wearables that even its smartwatch does not have many "smart" functions. i.e. it's not a competitor of Apple Watch. By straying into the fitness band category, Apple will have to start competing with a lot of wearables like these, which are frankly more health-specialized than anything Apple has the willpower to produce. Right now, the Apple Watch only has three competitors.
So if there was Fitbit and an Apple version which was guaranteed to be fully integrated into their new Health Ecosystem for a few more dollars, you would shun the Apple version and think everyone else would also?