The only way the Steam model would work for media is if content owners would agree that if someone on the platform purchased a digital copy of something that the ownership couldn't be rescinded.
Nobody will purchase, for instance, all the seasons of M.A.S.H. if they know that there is a risk that the rights owners will pull their content off of the platform in a few years and the person loses the ability to watch what they paid for.
The platform should also have the ability to "import" previously purchased rights. If someone buys a TV show on iTunes, they should be able to import that purchase into the platform so they can consolidate all of their purchases into a single platform.
Lastly, the other thing that made Steam a huge success is the sales. Daily sales of some content, quarterly sales of large swaths of content, etc. That would have to occur for a media platform to have the same kind of success.
I'm an unabashed pirate because it's easy... but if I could use a platform like this that didn't have a recurring monthly cost I would use it.
If Pixar put their upcoming movie available for pre-purchase (like Steam Greenlight) at 40% off, I would likely pre-purchase it. If Disney wanted to drum up interest in their new Duck Tales reboot by putting the original series up for sale for $20, I would be all over that. A TV series in its 4th season with struggling ratings could put up a flash sale of previous seasons in order to get new viewers to binge watch before the new season starts. A TV series could also sell a season pass BEFORE the new season starts for an infusion of cash and release the episodes on the regular schedule.
A steam-like platform could work wonders for media consumption, but it'll be hard to get companies to do it.
Steam took off because it was required for an awesome video game that Valve released. Eventually Steam was more valuable than the game.
That's the only way something like this happen for media. A small studio with a hot property could build the platform for their own offerings but invite other media owners to participate. Adult Swim or SyFy or FX could probably pull it off.