No rulers present when opening legacy files pre-Illustrator CC 2019

This is probably something that I'm going to have to live with but, since installing Illustrator CC 2019, every time I open a legacy file that hasn't been created in 2019 (which is everything I created up until a week or so ago) none of my rulers are present. I have to switch them back on. A minor issue but frustrating, nonetheless, bringing us to the age-old question of why can't they be switched on constantly in the first place?