Yes, you are correct. Along with this, aging, temperature, friction, and other factors all affect O-rings. In fact, the government writes PMS (preventive maintenance schedules) to manage this sort of thing. My point is O-ring designing is so well understood in engineering that it shouldn't be the issue to ground a fleet of helicopters. Prototype testing shouldn't permit it. The same with the Columbia space shuttle, which wasn't the maiden bird that flew. I find it to be a 'convenient' cover for some other reason to ground a fleet of birds like this.