The horizon model originally came from the book The Alchemy of Growth by Stephen Coley, Mehrdad Baghai, David White. The purpose for the model was to help businesses think about their need to develop new revenue sources over time. Near term, you must protect your existing revenue sources and if possible, even extend them. But at the same time, you must consider products or markets that are in decline and how you might replace that revenue. The trick is, you have to start building the replacements today in order to realize the revenue in the future.