If Uber pays drivers more than Lyft to a point where drivers stop driving for Lyft, riders won't be able to get a ride on Lyft, so they'll pay Uber's higher prices. Not everyone will; some will opt for a regular taxi, driving themselves, or transit. But I suspect that there is actually some combination of higher ride prices and higher driver salaries that will keep Uber alive and growing, though probably not at the same rate.