That's true. But there's a clear existence proof that there is a market for people willing to pay what it costs to drive them around. It existed before Uber and Lyft and it still exists alongside them. Certainly there's a price sensitivity curve but there's no reason to believe a national on-demand service like Lyft or Uber couldn't operate profitability in the absence of being undercut by VCs burning money.
I.e. while someone earning $200K might decode they can afford $20 rides rather than $10 ones, someone making $50K might think differently. The "who cares" when people go back to riding the bus is Uber.