I would totally download an app that listed people in my area beneath my stated weight threshold who wanted piggyback rides.

However, it should prevent you from changing your weight threshold more than once every 2 days. Otherwise, you'd be able to binary-search to find random people's weights.

Yes, but eventually Uber Walk would be rebranded Uber Walk Premium and a new service under the old Uber Walk name would be started where you get a 50% discount for walking yourself.

You can hail hand-pulled rickshaws in a couple cities in India, which seems pretty close to this. There might even be an app for it.

I saw (on TV) a guy who charged to walk "with" you.

not novel enough; the romans did that thousands of years ago