Only if you are in #1 in a pie that isn't getting bigger. Ridesharing is most definitely not a developed market globally. There is a lot of room to increase the size of the pie.

Ridesharing is definitely not a developed market globally because in most of the rest of the world the margins on organising transport around cities are already measured in cents rather than dollars. These markets are not going to be more profitable for Uber than the ones they started in.

Particularly not if you can't set up an operation in an Western city with expensive taxi services without losing money on every ride you operate even before centralised overheads are taken into account.

Is there evidence that Uber is has negative gross margins in developed Western cities? (Not a rhetorical question - I'm open-minded and very curious.)

From their 2015 financials, it looks like revenues have been consistently higher than cost of sales.

Source: http://www.nakedcapitalism.com/2016/11/can-uber-ever-deliver...

Is there evidence that Uber loses money on every transaction?

According to financials from 2015, they have higher revenues than cost of sales:

www.nakedcapitalism.com/2016/11/can-uber-ever-deliver-part-one-understanding-ubers-bleak-operating-economics.html

This suggests to me that marginal rides are profitable, even if they aren't profitable enough to cover fixed costs of software development or investments into new geographic markets.