I would disagree. If you only do X, in fact I think you will miss a lot of things that could make you better. You can become better writer by reading other great writings, if you only write yourself, you will not have the full big picture on what is possible. Then you can become better by thinking a lot, imagining a lot, etc... Same with most fields I would argue.

Although we were discussing about the decay of skill in something. While in some things the decay is super clear (as in running - pace, not the technique), I think there's many areas where there's no clear decay and other activities will actually significantly boost it, and any decay that there is, will be removed in just few days of practice or remembering.