Restoration using traditional and well-documented techniques are perfectly "authentic".
Except people don't want "authentic", they want what they know. No one is suggestion restoring the pyramids to their original look because then they would look 'fake' to most people.
You see it in more mundane situations as well. If a houses was red for 300 years and then white for 150 years, and someone wants to paint it red again a lot of people will complain because that building has 'always' been white and red will feel inauthentic, even if you can prove with historical documentation that red was the original 'authentic' color.
Except people don't want "authentic", they want what they know. No one is suggestion restoring the pyramids to their original look because then they would look 'fake' to most people.
You see it in more mundane situations as well. If a houses was red for 300 years and then white for 150 years, and someone wants to paint it red again a lot of people will complain because that building has 'always' been white and red will feel inauthentic, even if you can prove with historical documentation that red was the original 'authentic' color.