I'm guessing Isaiah was written maybe 300 to 1K years BC. (It's been a long time since I had any interest in the subject.) You're saying you think we have ORIGINAL text - text that was written by the authors themselves and not just stuff that was copied from scribe to scribe?

Or are you saying we have stuff that was copied and recopied and re-recopied, but which we believe is pretty close or identical to the original?