There is no doubt that to get better at running, you need to run more! However, it isn't possible to run all the time.
There comes a time when it might be better to do 45 minutes of cross training, rather than that easy run.
Elite athletes will add in underwater treadmill sessions into their weekly training, this allows them to do more running, but reduce the impact. Unless you've recently won the lottery, I don't think installing an underwater treadmill at home will be suitable!
If you struggle with injuries or are just coming back from an injury, then cross training over a run would be smart
If you are a bit frustrated with your running, and mentally are struggling, then cross training over a run would be smart
If the weather is bad, you are in a new location and don't know where to run and its dark, or you are stuck in a gym and don't fancy a treadmill, then cross training over a run would be smart
If you are a beginner, you'd be better off on the cross trainer at time, especially if you are considering training on back to back days
If you are looking to add some training volume to your week, then why not cross train
Why not add some cross training in after your run, so do 40 minutes of running and get on the elliptical trainer for another 20. Suddenly you've done 60 minutes of exercise, but only loaded the legs up with 40 minutes of running.
There's plenty of times the cross trainer is suitable, in fact for one run a week it really isn't going to be an issue if you cross train rather than an easy run.
Cross Training and Easy Running can replace each other very easily, but when you have a session to do it is much better to do this as a run rather than cross train, because we need you to practice running faster, and you just don't get that leg turnover and specific strength from cross training.