So even with those ultra unrealistic assumptions (100kg people, 1 step per second, 100% efficient energy capture), 9.8 watts just isn't enough.

Lighting needs about 0.6 watts per square foot (6.46 watts per square meter) in an office. That means you need someone like that generating 9.8 watts every 16.3 square feet or 1.5 square meters.

There's an inherent tension there, where sufficient density to make that work would require people to take fewer, shorter steps.

A basketball court is 4700 sq feet (436.6 sq meters). That means you'd need 288 big people stepping that fast, jammed into a single basketball court sized space, just to keep the lights on in that space. If any of the people stop moving even for a second, the system fails to keep up.