"The question would be whether Uber, Zipcar, Etsy, etc are taking any more of a cut than a a more traditional taxi medallion, a car dealership, an art gallery, etc. I suspect they are taking less, since they are transferring a lot of the risk of owning the property and running a business to their contractors."

I would think the opposite. What makes you think the companies wouldn't take maximal profit and make their contractors accept increased downside risk at little-to-no increased net income?