Definitely a business decision, although their thinking isn't "to continue to push people on to Win10," even though that's the end result.
The real business decision is to cut extra development, test and support costs and save a lot of money as a result.
It costs a lot of money to develop software on a new hardware platform that's running your latest software platform/product, and then have to spend additional time and money back porting and sometimes making design changes to then also make the new code run on older software platforms. More work, more testing, more cost all around.
I personally don't blame them from a business perspective. Having to design, test and support new code & features for 3 different operating systems vs. just one will no question cost a lot more money and people resources.
... and yet somehow you can run Linux on anything from a home router to the largest supercomputers on the planet. I think crying poverty won't really go over well with most folks for a company like microsoft.