Can you describe the steps that would lead to Uber becoming a monopoly? I can't imagine that happening without government help, can you? And if it requires that kind of help, is the problem with Uber or government?

If they continue to evade law enforcement and ignore the laws that taxi companies follow they will have an unfair advantage over taxi companies. Taxi companies have and will continue to go out of business. If you follow this reasoning to its logical extreme you have an Uber monopoly.

That might be bad for taxi companies, but what happens to Lyft in that scenario? Won't other ride sharing companies start up?

Uber only becomes a monopoly if they can keep all competition out, not just taxi companies. Taxi companies succeeded in doing that for decades by colluding with politicians to limit competition. Uber would have to adopt a similar strategy to get anything close to a monopoly.

If Uber ever reaches a saturation so high that much of congestion is made up of ubers, they will literally be blocking access to potential customers. An uber will always be near, but an "unter" won't make it to pickup in time, because of all the congesting ubers.

Driving as a service always operates on public infrastructure and medallion systems are there to protect those commons from excessive use. If uber does not like the rules of road use, they are free to purchase their own right of ways. It's America, nobody will stop them if they show up with enough cash. Also, no more speeding tickets for uber drivers!

I don't see anybody saying that corporations shouldn't be held liable for doing things which cause specific, identifiable harm. Like, for example, dumping toxic chemicals in the water.

But who holds the State accountable when they do the same thing?

That's what checks and balances are for. Ideally it'd be the judiciary, if it came down to it it would be the people.