Windows upgrades have never been a major source of revenue for Microsoft, and the company's shift to the cloud has made that fact even more apparent. I wrote about this phenomenon last year, in "Microsoft's transition from traditional software to the cloud is picking up steam." Here's an updated version of the chart from that post, covering Microsoft's full fiscal year 2015, which ended a month before Microsoft's free Windows 10 upgrade offer began.
Contrary to what you might have heard, Microsoft isn't giving away Windows 10 for free. In the six months ending December 31, 2015, OEMs paid Microsoft more than $6.2 billion to install Windows on new PCs, with that amount including tens of millions of licenses for new PCs running Windows 10.
Meanwhile, in the last year for which we have figures, Windows licenses sold through retail channels, which includes upgrades and academic editions, constituted 8/10 of 1 percent of Microsoft's total revenue for the year. That's a mere drop in the bucket