Because eventually you won't have a choice. That's how Microsoft works. Newer versions of Office come with slightly different file formats so people using older version have to upgrade. There's no plugin for new format or just degradation of the document when opening. They outright refuse.
Microsoft pushed Windows7 in similar way. New version of DirectX supported only Win7 and not older versions, even though there's no reason not to from a technical point of view. But new games supported new DirectX only and if you wanted to play better shell out those bucks.
In the end, biggest enemy to any paid software is not open source competitors, it's previous versions of their own software for the very same reason you mentioned. Why would anyone upgrade if all they need is already there. Most people don't need all the features of Office apart from different fonts and sizes, perhaps occasional table.