I have raised worms on cardboard alone. The trick is to get the cardboard reduced to a small enough size that it is consumed quickly. To do this I had to design my own shredder as all the commercially available units either cost too much or do not process enough. This food source is free and can easily be collected locally. See my cardboard shredder at: https://www.youtube.com/watch?v=B9JHbRpwbwQ
I also raise rabbits and use the ground cardboard for nesting boxes. I use the poop as worm food with the cardboard.
Hello Mark yes you are quite right cardboard is a great worm food and there is nothing wrong on raising your worms on cardboard alone. We always try to use worm food that is freely available. Worms love especially corrugated cardboard.
The combination of worm farming and rabbit farming is excellent and I am sure that you will recycle loads of organic material this way.
Your cardboard shredder looks like a very useful machine to have.