It's a low-skill job that just about anyone can do. Almost all jobs of this sort will be eliminated with technology/automation ESPECIALLY since higher labor wages put upward pressure on operating costs.
i.e.: once self-driving vehicles come onto the scene (as in ubiquitous) there won't be a labor force to worry about.
This would make more sense if a $15/hour wage was something out of step with historical wages. It's a big jump right now, but historically it's not out of step with previous periods. What were average wages at taxi companies pre-Uber?
As for self-driving, I'd be a little concerned that a ~$8/hour cost (assuming previous was around $7) puts Uber in an untenable situation, self-driving cars or not. Ultimately there still would be labor force to worry about, no? Currently, drivers are responsible for maintenance, cleanliness, etc. -- once you switch to self-driving, someone else has to do all of that.
Comparing them to taxis isn't one-for-one for this context: most municipalities had barriers to entry to prevent just anyone from running a taxi service (i.e. the medallions in NYC).
The business will do the math to figure out what they can get away with. Honestly there probably is a little political posturing here as OP said, but if they return it's because they made the math work, otherwise they'll stay gone.
I can tell you with certainty that most low-margin, high workforce industries (because I work in one of them) are looking to automate much of their workforce away especially in light of labor costs rising (minimum wage laws), increasing safety regulations (let the robots do it), increasing competitive pressure (foreign countries have cheaper labor), and social changes (people prefer interacting with a cell phone over a human being).
Couple this with a publicly traded company such as Uber/Lyft whose whole existence depends on market value (the stock price) then the management of those companies are going to do the math to figure out if automating jobs is the right thing to do: their job is to maximize the value of the company (the stock price). The shareholders also have a vested interest for the same reasons.
i.e.: once self-driving vehicles come onto the scene (as in ubiquitous) there won't be a labor force to worry about.