Well, to keep it in the analogy, this is what the world without nn sounds like - "I bought a economy ticket but I am reaching lot late than the one who bought a coach ticket." It's not that I am getting a tight seat or shitty service. I am reaching late to my destination. That means eventually everyone would have to pay more depending on the whim of the carrier.

Express service on Amtrak costs more than the slower service.

It's not the whim of the carrier. It's what the carrier can negotiate. Why should the US Gov't be part of that negotiation?

But if I don't mind arriving later to pay less for a ticket, why should I be prevented by the State from doing so?

The issue isn't the speed, it's what this arrangement can lead to. I believe the inherent right to information supersedes the wants of a few corporations. What we are really talking about is potentially losing access to valuable information. To me the internet is a public domain and limiting traffic like this is the equivalent of limiting speech.

But the internet is not a public domain; most of the infrastructure was built and paid by private actors.

If society considers it a real priority, should an actual public ISP be built, instead of forcing rules onto others'?

Public ISP would be a great idea. After all it is just a utility.

First class and coach still arrive at the same time though. That isn't the case with different levels of traffic.