But Juno is only available in NYC (for now?). I think my points still hold outside of densely populated urban areas.
Sure. There are density thresholds for any local businesses. I can't even really get Uber around where I live. Maybe there are a few scattered cars but not many. You'd be pretty silly to set up a competing ride-hailing service here.
There's probably some critical mass for these services to work well and that's going to have at least some effect on whether it makes sense for a new entrant.
If you subsidize driving and subsidize riding significantly, people will sign up. A $40 bonus covers several rides for a customer so they'll hold onto the app at least until they've used their initial free offering.