I used to think the dependence on line was the biggest differentiator between fine art and illustration.
Then I thought it was the utility. Illustration is a tool to depict a vision, decorate a web page, accompany words for emersion into a story or article. Fine art’s only official utility is to be useless for anything other than appreciation. It’s a unique object in that way. Unofficially, of course, fine art is extremely useful for increasing wealth for collectors.
The only difference I can come up wtih that holds up to scrutiny is a contextual difference.
A piece is fine art when it is in a gallery, museum, or collection or made with the intention of showing in these contexts.
A piece is illustration when it is almost anywhere but a gallery, museum, or collection.
Christian Robinson immediately comes to mind.
Christian Robinson makes fine art that is often used in illustration contexts like children’s books.
Take a long look at Robinson’s work and tell me it isn’t fine art.
If it didn’t depict kids, cute animals, etc, lets say those smiling faces were erased, would it not be incredibly well-considered abstract art?
His education oozes off the page. He knows exactly what he’s doing and you can see his links to art history clearly on every page.