Running is not the only activity that keeps heart trained and engaged, but it is always an easy way to do it and an important comparison to other cardio activities.
I can confirm my running standards stayed the same without running, but doing other cardio activities such as spinning and rope jumping. Running is always fine to control the overall body functioning, for example to check digestion and belly, to check articulation resistance, to check general endurance, to check the weight of your body mass. When running, you feel the weight of your body. Each inuseful kg has a specific impact. This does not happen with swimming and bike, that are very gentle on body mass. Running is harder as far as body shape is concerned.
I find that rope jumping is simply great. A bit heavy on the spine and challenging to articulations, it shapes the body better than running and any aerobic or simple step lesson. Rope jumping requires coordination, breathing and endurance. However, rope jumping is always more gentle than running as far as general body weight is concerned. Running gives you the measure of how fast and how distant you can “escape”: running deals more with survival than other activities. Psychologically, everybody would need some running. People who never run in their life are usually people who have a very comfortable lifestyle or a comfortable set of rules to be respected.
For these reasons, I integrate methods and suggest some running also to non running people to better feel the body and enhance it, completely.
At the same time, I say to runners that running might not be enough if the goal is a well shaped body.
What matters is the goal. Why do you train? What would you like to do with your body? How would you like to shape or enhance your body?