Does semi-modern auto glass get brittle as it ages?

I made an appointment and took my older (1978) Mercedes to the dealership to have the rear window gasket replaced. I got a call this afternoon they don’t want to do the job because they were afraid they might mess up the molding and break the glass. Does auto glass become brittle with age, or are they just scared to be working on an old car? Its the dealership for cryin’ out loud!