I know that it's true for me, at least, that I become appreciably dumber when I do less research. As with other areas of life, things are usually improving or deteriorating in my mind, rather than in pure stasis. I am either becoming a better writer, or a worse writer, at any given time. I couldn't imagine still teaching the information I learned in my Graduate Program, which I finished in 1988. That stuff would be incredibly stale by now.
For me, just reading or "keeping up" doesn't do it. I can't keep up my scholarly base just by mostly passive consumption of primary texts, or the scholarly work of others. I need to be engaged actively and urgently in the production of thought. Of course, the passive consumption still occurs, but for me it is not enough.
I realize that everyone is not like me in this regard. Other people might not need to be on the "edge" all the time, or they may not fear losing it. Some don't have the psychological need to feel smart, or to stretch their brains constantly.