It never "seemed to be sustained". It was always and obviously an artificial result of vc funding designed to get a market going and obscure the economics until it's established enough that they can get away with changing the deal like uber.
It's illogical to ignore the size of the market and think that there should somehow be a need for just any number of drivers instead of the required number of drivers.
It's illogical to ignore that when prices to customers are artificially low, customers will consume more than when they actually have to pay for what they consume.
Since the driver pool has been paid for by vc funding rather than consumers, there are more drivers than needed. A smaller number of drivers is perfectly sustainable.