No serious scholar ever doubted the existence of Isaih.
For many unbelievers, it will just be another piece of evidence that they will fail to consider or flat out ignore. For other unbelievers, they will just want more.
John 12:37: “But although He had done so many signs before them, they did not believe in Him.”
I was thinking the same thing. He was pretty much a “rock star” back in the day.