The idea is that while it is nice to enjoy subsidized rides at the moment, as a consumer you may suffer in the future due to Uber establishing a monopoly.
Uber has no moat, so cannot possibly exploit a monopoly. The hardest part of their service to replicate is the drivers, and since they're all contractors they are free to work for Uber's competitors!
Even if Uber gained 100% share of car rides in an area, if they decided to exploit that by increasing their prices it would be relatively easy to create a competing company and undercut them.
Uber can only compete on price, they have nothing novel that can't be replicated. Hard to imagine how they could control the supply in a meaningful way.
Even if Uber gained 100% share of car rides in an area, if they decided to exploit that by increasing their prices it would be relatively easy to create a competing company and undercut them.