That was my first thought as well. Accurate navigation from footpod data that could eliminate or at least drastically reduce gps usage would be a gamechanger for watch battery life.
The sampling rate you need from the inertial sensors is such that I'm not sure it's likely to use much less power than the GPS for similar accuracy.
Interestingly, both my Stryd foot pod and Garmin 935 watch (with GPS active) last about 24 hours. The Stryd generally provides more accurate distance/pace measurements. At least in one person's testing, the Stryd is the most accurate device currently available for distance/pace: