Where did the "Slash and Duff will just re-record some stuff from old CD leftovers"-talk come from? Just fans going down a spiral turning it into some accepted reality -- or has any band member said that they will go back using Axls old stuff?
I would think the odds are greater that Slash, Duff and Axl starts from zero or go back to the Illusions or Appetite sessions to find something, and not that Axl comes dragging in 20 year old songs from the other band asking Slash to replace a solo but keeping the vocals etc.
Hard School being soundchecked, might be an argument against that ofc. If