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.