Generally true, but I wouldn't shy away from an airplane that's been covered in Grade-A. Depends upon a lot of factors. My Grandpa's cabin Waco was covered with Grade-A and it lasted from 1967 until it finally failed the punch-test in 1987-88. The airplane was kept in a hanger, with the exception of residing outside at numerous flyins for weekends. The airplane was dark-green, to the point of looking black at a distance greater than 10 feet - and dark colors show wear and aging worse in my view. After 20 or so years, the airplane started to get some checking and cracks at various spots, but still looked incredibly good and "wet" (it had a 30 coat, hand-rubbed dope finish). Generally you are absolutely correct, but cotton CAN be made to last a long time. He also bought a Chief that was covered in Grade-A, and it was doggy looking when he bought it in 1974 - (think fabric the consistancy of a tennis ball - fuzzy) - but it passed punch tests until 1986 - and I don't know how long it had been covered by the time he bought it. That was a handy airplane because I learned how to patch fabric and got lots of practice at it. Grandpa said it was a novel approach and if the plane had passed a couple more punch tests I could have basically recovered the entire machine - one patch at a time.
