If it were legal to download movies and music, Netflix and Spotify would absolutely not exist.
Steam is an unusual case, because games are running software and can't be trivially reproduced in their unencoded form. The publishers can include copy protection, network connection requirements, or even run essential parts of game logic on their own servers. So free downloads became a much worse experience over time.
Netflix might accidentally be the worst possible example you could pick for 'wouldn't exist if piracy were legal.'
Netflix built their entire streaming business model during a time when piracy was so widespread it was almost as good as legal. They succeeded precisely by proving that people would pay for good service even when free options were readily available. They're a textbook example of a business that thrives by being better than free!
Despite huge investments in enforcement, movie piracy never waned. The reason it declined? Netflix. Why is it now seeing a bit of a resurgence? Also Netflix, actually, or rather the fact that people have splintered the streaming landscape.
Here's some articles from Forbes at the time. [1] [2] [3] and an interview with the Netflix CEO [4]
People following the Netflix/piracy story at the time saw it like this: Netflix doesn't necessarily need to care if piracy is legal or not, because it removes most of the incentive to pirate. People tried a lot of things against piracy, until Netflix came along and that was the thing that actually worked. Piracy goes down where Netflix is available. I've also provided enough sources to explain why: Piracy is a service problem [3]. Netflix provides the missing service, so people don't feel like they need to pirate anymore.
In a world where piracy was fully legal, Netflix would still exist, and still drive down piracy. This is Netflix's entire reason for success!
I think you're arguing in good faith, so my response is that I think the legal-piracy world would make it very easy to download things, on par with going to YouTube, something anyone could and would do. I don't think Netflix would be the same in that world. They might well have been the premiere free movie distributors!
We have an internet. It was and is already very easy to download things, legally or not [1] . That's why speculating about legalization making piracy easier misses the point - it's already as easy as it can get.
Netflix just made it that much easier to find what you want[2] and just watch it. People were and are willing to pay for that.
[1] for movies and other large files you can use a torrent tracking site: type what you want, click the result with most seeds, download, play.
[2] Not necessarily as easy as it used to be a few years ago. So piracy is going back up.
Steam is an unusual case, because games are running software and can't be trivially reproduced in their unencoded form. The publishers can include copy protection, network connection requirements, or even run essential parts of game logic on their own servers. So free downloads became a much worse experience over time.