Would you mind elaborating on the idea of sustainable pricing?
I ask because it sounds not too far away from, for example, "With the goal of artificially inflating the price of transportation, for the benefit cartel members, we will constrain the supply". See OPEC. This type of collusion is typically considered to be anti-competitive, and not in the best interest of society overall, or of the market counterparties. Silicon Valley's "No Hire List" conspiracy is another example of cartel action to manipulate market price for the cartel's benefit - in that case, illegal.
Why should the government support a specific type of private business (taxi companies) in this way? Competition is a good thing - we ought to encourage taxis and other transportation companies to compete with each other and offer the lowest price. I don't in general want my government to prop up businesses that cannot survive on their own (moral hazard, among other reasons), especially if society can solve the same problems with businesses that thrive naturally. To make this claim, there needs to be an objective way to define what a "sustainable price" is. Can such a thing be defined? (except as follows)
The fair market price is the sustainable price. Anything else is unsustainable and will only persist as long as artificial constraints are applied, such as arbitrary limitations on supply. This is not healthy, and harms the market and the consumer (deadweight loss). A functioning market balances supply and demand naturally through the negotiation of price.
Do other forms of transportation such as long-haul trucking or air travel require limitations on the number of trucks or planes in order for their markets to function effectively? It seems as though they don't. We expect airlines and trucking companies to compete with each other in order to offer the lowest price, and these days consumers select airlines largely on price. Why is regulation to artificially constrain supply necessary for taxis but not trucks or airlines?
It seems to me that such regulation is in fact not necessary, and not in the best interest of society; but is rather regulation that has resulted from "regulatory capture", and protects certain vested interests, at the expense of other transportation companies like Uber and Lyft, as well as the general consumer.
Surge pricing is a good thing, and if regulations prohibit it, then that's another example of undesirable regulatory intervention in a market. Surge pricing is another concept that's effectively standard and well-accepted in air travel: if you travel when it's busy, the fare will cost more. I discussed surge pricing in a previous comment: https://news.ycombinator.com/item?id=8720969 The consumer protection issue of fixing taxi fares so that people don't get taken advantage of by unethical cabbies does not apply to Uber.
I am not sure about the insurance issue, but I agree that Uber ought to be required to provide whatever insurance is conventionally required by vehicles in that category. Perhaps that's taxis when comparing to UberX, and private towncars when comparing to Uber.