Consider that both rollerballs and, to a lesser extent, ballpoints have a spring that is constantly exerting pressure on the two ends of the body of the pen. Should it surprise us when one of those couplers is loosened over a period of years? I really don't think so.
Your post made a lot of sense to me when I first read it. However, as I thought about it, I'm not sure that I can agree with your logic.
I don't believe that a spring that cannot push out a coupler today is ever going to apply enough force to push that coupler out. Further, if the spring were able to do so, what you would find on your shelf is not a pen with a loose coupler. You would find a pen with a coupler that had been pushed out of the tube.
An analogy would be if I went into my garage and attached a strong bungie to the wall. I then stretched the bungie and attached the other end to my car. The bungie isn't strong enough to move my car, so nothing happens. I measure the distance between the wall and my car. Once a week, I measure that distance. It never changes because the bungie never becomes strong enough to move the car. In fact, the amount of force that the bungie applies actually decreases over time because the bungie wears out.
The same thing will happen to the spring in your pen. If the spring wasn't strong enough to move the coupler when you made the pen, it is never going to get strong enough to move it.
Here's another theory as to why you are finding loose couplers on those old pens: They were always loose.