When talking about software products like Microsoft Office, the usual approach is to buy them once and upgrade in 5, maybe 10, years. It comes as a natural approach since the core functionality has been there for a long time. Of course, each future installment brings in new features, tools, UI updates, improvements and so on, but all this new content is far from being critical. Many people still use older operating systems like Windows 7 and office software like Office 2010.
I think this was the main driving force behind the decision to make subscription model more attractive. On a long run, like 5 or 10 years, the subscription-based model gives you more profit compared to one-time purchase in a market where many of your customers prefer to buy your product once per 5 or 10 years.
Also, the subscription model is more beneficial for customers as well. It is very flexible – you don't have to be subscribed each month and pay a large chunk of money. Add to this features that subscription has and one-time purchase doesn't, such as latest features and updates for free; up to 5 installations on different computers; extra Office features for tablets and smartphones; 1 TB of OneDrive cloud storage per user, for up to 5 users; and full technical support. Clearly, even if you know for sure that you are going to use Office for next 3 years, the subscription model is better. The price difference and all the additional features you get are more attractive.
If already made up your mind, you can buy Office 2016/2013 Pro key on GamesDeal.