Essential means technically and financially essential. I.e. without it the service can't work or can't bring profit. DRM is neither of that. Those who require DRM [publishers] have no technical or financial reasons to justify it. No valid ones at least (all reasons they usually voice are false, and their true reasons they usually don't voice).
1. Monopolistic lock-in. DRM is more than often used to control the market. It happened with Apple in the past, and was one of the key reasons that music publishers realized that being DRM-free is actually better for them.
This reason also includes DRM derivatives like DMCA-1201 and the like. It's all about control (over the markets, over users and etc.).
2. Covering one's incompetence. DRM is used to justify failing sales (i.e. when execs are questioned about why the product performs poorly, they say "Pirates! But worry not - we put more DRM in place").
3. Ignorance and / or stupidity (many execs have no clue and might believe that DRM actually provides some benefit). This type can be called DRM Lysenkoism.
None of these reasons are valid, all of them are crooked and anti-user, but they are often present in various combinations.
How does this benefit the publisher? They already have a exclusive rights to whatever particular movie you're watching.
When publishers realize that monopoly falls to some other hands, they quickly become sober and find common sense. When monopoly remains theirs, they pretend that DRM is needed for other reasons.
A turnstile isn't very hard to jump over. If the turnstile is the only enforcement mechanism, anyone who wants a free subway ride just has to vault over (or perhaps also glance around to see if there are cops nearby, google whether there are monitored cameras, or pull a hoodie over their face).
There's an alternative, though: most places in Europe, there are no turnstiles, and nothing preventing you from getting on public transit without paying. It's a "trust but verify" system: you are required to validate your ticket before you get on the train, and occasionally someone comes around the train to check if you have a valid ticket, and issues a fine if you don't have one.
With this method, the users aren't inconvenienced, enforcement costs are reduced, and everyone's happy. With the turnstile, you get massive queues at busy stations during rush hour, and by making this one mechanical device your sole point of enforcement, you've actually made it more likely that people who don't want to pay will cheat the system.
That's quite a false dichotomy, since the service clearly can exist independently of DRM (both technically and economically).
Furthermore, this dichotomy doesn't work either in your analogy (there are ads on the subway, and yet I still have to pay) or in the real case we're talking about (Hulu+ is paid service locked down by DRM, and it still shows advertisements).
It's not just reasonable, it's easily demonstrated in practice, when goods released with DRM are pirated almost right away (with DRM being scraped off). Sophisticated pirates are needed to break DRM. Unsophisticated ones get it from them without any hassle. Which means that DRM has no effect altogether except on legitimate users!
I.e. the vast majority of pirates never deal with any DRM, and experience of legitimate users is always degraded by it. It's pure common sense that removing DRM would always improve any service. And the fact that it's not happening is caused by other reasons (see above).
There's no reason a BitTorrent movie downloader couldn't be made just as easy to use as a Netflix downloader. If one doesn't already exist, it's not because DRM is somehow stopping it.
About renting vs. sales see https://news.ycombinator.com/item?id=7745797
Similarly, some places don't use DRM. I don't think it's made it into the video space yet, but legitimate DRM-free music is common. It seems to work pretty well.
To say that DRM enables the service to exist only works if there's no other, better way. But there is, and the only reason we don't get to have it is because of customer-hostile policies from media companies.