I'm inclined to think, from the sounds of it, you're more likely to damage the cables by attempting to move them and putting a grommet on. Clearly they've been fine for 30+ years.
Quite often there's plaster around the cables where they enter the box which prevents a lot of movement. But even if not, the sheath on those old cables was actually more resilient to rubbing on edges than most of the modern stuff. If you can add protection (in the form of strip grommet or whatever) without too much difficulty fine, but if not and there's no sign of damage, I wouldn't worry about it too much. As has been said, how often are you pulling sockets out anyway?
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