Sony deletes more movies from the accounts of people who ‘bought’ them

It destroys the library model used by Spotify, Netflix, and all the other similar services for one example. Those are clearly not working on the same basis as selling permanent copies of everything you might want to listen to or watch. They clearly do make a lot of revenue from subscribers enjoying the long tail of music and programmes and often that includes repeats. Many more people enjoy many more works that aren't the big headliners under this model. People can also try something they might enjoy without committing to the cost of buying it and therefore don't have to feel bad if it's not for them and they give up after a few minutes. And yet obviously the subscribers individually spend far less in many cases than it would cost them to buy permanent copies of everything they'd listened to and watched. Given the popularity and financial success of the streaming services this is evidently an alternative model that works for both sides. So I would challenge your claim that the loss from always providing permanent copies is insignificant.

I don't really buy the other argument you're making either. With digital works there isn't much reason for a "second hand" market where copies would be significantly cheaper than a "new" copy direct from the supplier. When people used to trade used works on physical media there was a degree of degradation in those media that justified a price reduction. Why would someone who had bought a copy of the latest summer blockbuster sell it for 30% of its original purchase price if it's a flawless digital reproduction identical to a new one? Naturally this shifts the dynamics in the market and the price of buying a true permanent copy that can legally be sold on afterwards would tend to increase because of this effect. Meanwhile the library services I mentioned above work on almost the opposite basis and it tends to push the price per work accessed down because the subscribers aren't effectively subsidising other people who are enjoying identical works to themselves but without paying the original source anything to access those works.

I think there is demonstrably room enough in a world of billions of people with access to orders of magnitude more content than any of us could experience even once in our lifetimes for multiple economic models. What matters is that people get to create useful works and other people get to enjoy those works and the financial arrangements make this worthwhile for everyone. There are certainly flaws in the current copyright model that is established in most of the world. There are rights that I think people who have bought (or believe they have bought) permanent copies of works should enjoy with the force of law behind them if necessary.

I don't have a great answer yet to the problem of rights holders not wanting to make anything available permanently at all so that everyone is locked into some form of temporary arrangement. Clearly market forces haven't always sorted that one out effectively and some sort of adjustment is warranted. But I'm also wary of relying on some form of government regulation that distorts the market and potentially excludes arrangements that everyone actually involved might find worthwhile. Maybe you could somehow require that once any work has more than a certain number of licensed copies in circulation or has been available to a certain number of people from authorised sources for a certain (relatively small) number of years then it must also be available for sale as a permanent licence - regardless of any other continuing and still legitimate ways to access it from authorised sources - but with some recognition that a fair price to buy a permanent copy that comes with all the associated rights today might be significantly higher than what these things used to cost in the days when physical media were required.

This is true. (It's true in every other industry as well.)

But the opposite side of that coin is that if you want people to spend the considerable amounts of time and money required to create new works that are actually any good then you need to have some viable model for compensating them that makes it worthwhile for them to do that. Whatever else you can say for it - copyright has been far more effective than any other model ever tried at the scale of human society in achieving that.

I do not think there is any online platform that can make such guarantees for movies.