> then they don't need to buy as much new content to satisfy their customers, and total spending on content will go down.

Why would they have customers in the first place if all they offer is reruns, which everyone else also offers? Streaming only old content will be a very, very low profit margin business.

> Nobody is picking Spotify over Apple Music or Youtube Music due to exclusives, because there are none; so putting the content into the public domain is hardly going to change things.

Creating and streaming audio is not a comparable business to creating and streaming video, due to the vastly different sums of money, and hence risk, involved.

But, also, people have to pick only Spotify/Apple/Amazon/Alphabet and a couple others because of excess copyright terms. All the old hits people want are controlled by Universal, Sony, and Warner, and so if your audio streaming business does not contract with those 3, then you’re dead in the water. Which means every audio streaming business, and hence every audio streaming customer, is always paying rent to those 3 businesses that own copyrights.

That means there is less money available for new audio creators. And this holds true for all rent seeking. If it weren’t for excessive copyright, there could be much more variety in audio streaming.

> Why would they have customers in the first place if all they offer is reruns

You can ask the same question for cable TV, but it's not dead. Netflix also started as purely "reruns" and was still quite popular.

> Creating and streaming audio is not a comparable business to creating and streaming video, due to the vastly different sums of money, and hence risk, involved.

If anything that extra risk should make studios more shy of investing in new content vs just serving up old hits. It's noticeable that film leans way more heavily on franchises and remakes already, which agrees with this hypothesis?

> That means there is less money available for new audio creators. And this holds true for all rent seeking. If it weren’t for excessive copyright, there could be much more variety in audio streaming.

You assume this, but I really don't think it's true! Most people don't seek out new music; their tastes are set in their youths and then they happily listen to the same music for the rest of their lives. The choice is to make them pay to listen, generating at least some stream of royalties, or let them listen for free, in which case they will be happy to.