You're misstating history. There are no "great works of art that are considered worthless in their time". They were considered valuable, if by no one else, by

You're misstating history. There are no "great works of art that are considered worthless in their time". They were considered valuable, if by no one else, by those that created them, and whoever happened to hang onto them, and pass them down through the ages. (After all, they still exist today.) But in almost all cases, it wasn't just the artists who considered their works valuable: many other people did as well. That's generally why they were able to continue to produce art. Patrons, who did appreciate their work, even if the general public didn't, bought it, and supported the artists.