The choice is not between Netflix and piracy. I can't speak for the original poster, but for me Netflix' appeal was that I had a wide selection of content available for a single price in a single subscription, making it an attractive competitor to buying and/or renting single movies and series. With the fragmentation of the streaming landscape they have lost that competitive edge, and if it wasn't for the Netflix-exclusive series that justify the monthly price for me, I would probably have cancelled too.

A streaming service is attractive when the monthly cost is less than what you would pay for the seperate content you actually watch, and if all the content you want is offered on that service. Needing to subscribe to multiple services can quickly balloon the cost and inconvencience to the point where it's easier and cheaper to just seek out individual content - either legally or illegally, the result is the same for Netflix.

Personally, I have no qualms avoiding paying 3.99 or whatever to rent a digital file that made its money back thrice over before it left theaters.

“You wouldn’t download a car” arguments aside - what is the ethical boundary for “They’ve made enough money - I’m entitled to enjoy it for free” ?

I agree, that's the kernel of the issue here.

TL;DR: I'm curious if there's a way to make effective ethical arguments when people have fundamentally different world views, including whether or not morals have any solid grounding.

You can judge a moral system by how society would evolve if everyone followed it.

If nobody pirated anything then we would still be buying music for 99 cents a track (or more) and music streaming would have been quashed by the labels.

On the other hand if everybody pirated everything then the music industry would have collapsed.

I conclude that the world needs all sorts of people.

I don't pirate anything, but maybe one reason people do is that getting things spoiled before you watch it can be a serious problem.