I see consumers not paying the true economic price as a form of stimulus. They're receiving more value than they are paying for, and so they presumably are living better lives and will be able to spend that saved money elsewhere (movies, food, etc). Similarly, if drivers are at less than full capacity that means there are more drivers than there would be without the funding, and thus more drivers getting paid.
No arguing against the environmental impact of course, other than the hope that cheap ride-sharing reduces the likelihood of car ownership.
