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The metrics they've picked are highly unlikely to change. Especially to steps, as steps can already be used to increase all 3 of the existing activity rings.

There's lots of third party apps that measure steps, however. Many already have Apple Watch apps that will display the data.
 
I am a 10k steps a day guy, but sometimes when I do certain workouts, I don't make it to 10k steps. I like the ring better. It accounts for the work I put in, even though I didn't just walk 10k steps on a particular day.
 
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