Walking is a weight bearing exercise, while cycling is not. That's why cyclists can develop osteopenia. (https://www.trainingpeaks.com/blog/why-cycling-is-bad-for-bo...)
The vast majority of middle-aged people who exercise at all, do only one thing.
You do you, but it's a bit like somebody consuming only large amounts of meat and somebody suggesting that it may be beneficial to reduce some of that and balance the diet with some other nutrients.
Health isn't my goal, it's a second-order effect. I just know that you can achieve top 1% fitness without the time-consuming yoga and gym routines.