You can look at a Kurt Schwitters collage and you can tell that those papers are really from the 20s, and they've aged, and the inks that were used on the papers have aged. This makes up what it is today. It would be strange to look at a collage done back in the 20s if you saw it like it was when it was made. You'd be disoriented. The colors would be real crisp and bright, and the paper would be bright white.
Paper just changes with time.
When you look at the paper, you see that it has aged over the years, and that actually makes it quite what it should be. It's lived an age, like a person who's 85 years old. You'd still have the problem that 70 years had passed. Something would look different from how it looked when it was made. Seeing things age is a form of beauty.