I don't think it's about Windows and Office becoming less important as concepts or products, but that their old sales channel and use cases are outmoded. Windows and Office will still be Microsoft's cash cows; they'll just collect the fees through Azure, OneDrive, and other software-as-a-service packages. Open-sourcing is about keeping Microsoft's platforms competitive so that people will rent Azure nodes.
Microsoft realized they were becoming to FOSS as OS X is to Windows and is trying to counteract that. Microsoft's vision is still valid; they want Microsoft technology running every computer in the world.
Indeed it's clear that Office is very important, they are just unifying products and services. Now you have a unified subscription of $ 9.99 per month for Office 365 + Office Desktop Apps.
Microsoft realized they were becoming to FOSS as OS X is to Windows and is trying to counteract that. Microsoft's vision is still valid; they want Microsoft technology running every computer in the world.