It costs some money to get a DOI. Publishers can still be content aggregators and indexers, or curators. Sci-hub is merely an archive.
Can't be too much, seeing as I can put a semi-unlimited number of PDFs up on Figshare and get DOIs for them for free.
At 4k per DOI, you can store 1/4 million of them per GB. They are basically MAC addresses for documents.
This is an active Machine Learning task in a lab at my university. Given the kind of progress they are making, I don't see publishers finding much monetary value in this task in the future.
All we need to do is give Sci-hub free reign to publish scientific articles, and see what happens.