I'm just sort of connecting the dots on my own.
- Sensor Core came out and it was a BIG deal. You saw it everywhere on WP 8.1. Apps were built around it.
- Band 1 came out... Then the Apps pivoted towards Band.
- Eventually around Band 2 the Apps morphed to all Band, all the time and Sensor Core was dropped from W10M.
- Band is now cancelled and Sensor Core on phones never replaced.
Seems like a clear case for what happened to me. They wanted Band to be their health / fitness / tracking focus so they dropped on-phone support. Band crashed and they simply moved on.
Meanwhile Sensor Core lies waiting to be tapped. It still appears in Settings > Privacy > Motion, yet nothing native to the phone will use it.