I think I need you to clarify what you mean. I'm going to take a shot at answering, but please let me know if I'm addressing what you're asking or not:
25-54 is pretty much impossible to win by targeting 25 year olds or 54 year olds. You need to hit the center of the target and let the ripples spread out 10 years (if you're lucky) in either direction.l
The longer KRTH (and for that matter, a lot of Oldies stations) held onto the 1956-72 stuff, the less appeal they had to the core of the demo. But they were afraid to make the move they needed to make because, since they'd lost much of the interest of the core of the demo, they might blow off the older listeners and have no one to replace them with.
It's fearful thinking, not logical and not helpful. They would need to advertise, promote, and rely on word-of-mouth and incidental listening for exposure to large numbers of the audience anyway...but, like someone trapped in a bad relationship, for a long time, they chose the devil they knew.
Bobby Rich, a great programmer back in the day and today, has a saying:
"Sometimes, you spend all your time grasping at straws when what you really need is one great big drainpipe."
In other words, sometimes, the big bold move is best.