Connection to the Internet is a commodity, like nearly everything else. Someone owns the network, the physical devices and has to maintain, monitor, repair and improve it.
It costs money and somebody has to pay for it. If one entity is responsible for 70% of your costs, and another 30%, but you are by law required to charge them 50/50, then it's not right. The 70% party is getting a bargain, while 30% party is getting ripped off. An ISP charging different parties different rates for cost reasons is the maximum of fairness. An ISP charging you out of spite is likely to be a non-event.
They are a business and they want more customers. The more people like to watch high quality Netflix, the more customers sign up with them for broadband.
Of course it goes the other way as well. If Netflix did pay more, and got better traffic for their service, then an ISP could sell tailored-performance packages for gaming, streaming, business etc. Alas, my traffic is equal, so I get average everything.
Indeed, your last example shows basic supply & demand economics. If more people cancel TV, the demand drops down fast, while supply is slow to drop. Prices fall down.
If more people sign up for online streaming and consume your broadband traffic, the demand rises quickly, while supply is almost constant. Prices rise.
The only way out of prices rising continuously, and from content providers getting charged whatever ISPs demand, is competition. The more ISPs you have competing customers, the less monopolies you have.
I use this fact every year, when I call my ISP to renew the contract and I say "Give me newcommers price or I will go with your prime competitor". And it works every time.
Your Slip N Slide situation is not analogous to this situation, as they are not the ones selling you the water. And the water park should get precedence if the contract they signed says "if you pay more you'll get more".