> Free market assumes that goods from different producers are substitutable
No it doesn't. A free market assumes that there are no constraining influences on trade except supply and demand. With the possible exception of a working contract law system.
Free markets happily allow for people to make buying and selling decisions for any reason they choose. If it happens to be for a non-fungible asset, no matter how pointless it may be to other parties, so be it.
Efficient markets imply restricting assumptions on fungibility, so that there can be a meaningful fair price, but it's not required in general.
I'm interested in your comment about copyright being proposed for stability. Can you direct me to a source please? If you have a reference or a hint for further research, I'd like to read into it more.
You're probably reading too much into that comment. In the U.S., the copyright was justified "to promote progress of science and useful arts", which I interpret in a way that it provides certain stability of income needed to promote learning and experimentation (and also allows for a failure). It was understood even then that it is an anti-free market policy.
And that seems to be what OP was complaining about, the lack of this security for artists. Unfortunately, the copyright (like most other property) today has been mostly captured by rentier class, so it no longer provides these protections for most artists, if it ever really did.
So I was really just pointing out this connection between OP's woes and copyright.
No it doesn't. A free market assumes that there are no constraining influences on trade except supply and demand. With the possible exception of a working contract law system.
Free markets happily allow for people to make buying and selling decisions for any reason they choose. If it happens to be for a non-fungible asset, no matter how pointless it may be to other parties, so be it.
Efficient markets imply restricting assumptions on fungibility, so that there can be a meaningful fair price, but it's not required in general.