I'm not sure about the two-horse race
With HealthKit and Google Health anybody can make a device that contributes data to and consumes it from the same store as everybody else*, and a lot of companies are doing that in modest but profitable ways to different niches — those are the Polars, the Garmins, and the Xiaomis. There's no need to have a blockbuster device, and if you look at Fitbit's result then it looks like it's not even necessarily workable to try.
* other than Fitbit, which likes to own your data for itself.