Cross-training, and not . . .

The transition to triathlon a few years ago taught me some great things about cross-training.  It allows us to increase training volume with less risk of injury.  It fights off boredom.  It exercises different muscle groups.  It also allows us to shift events when injured. All of these things are good.  One thing is bad.  There is a big difference between running 5-6 times a week and running 3-4 times a week.  The same thing goes for swimming.  I notice less of a difference with cycling, but FWIW, I'm more of a natural cyclist than the other two.  When I increase my focus on running, my running form improves.  Right now, I'm not running, so I've been swimming a lot more.  My swimming is improving.  I wonder if we shouldn't work more periods of training that focus on improving just one of the events, while deemphasizing the others.  Is it possible to improve, and then maintain the gain, or is it better just to keep rotating.  I suspect that there's not one right answer to this question.