On Microsoft's earnings call for the first quarter of its 2019 financial year, CEO Satya Nadella said that "more than half of the commercial device installed base is on Windows 10."
A Microsoft spokesperson "clarified" this to say, "based on Microsoft's data, we can see that there are now more devices in the enterprise running Windows 10 than any other previous version of Windows." That description offers a little more wriggle room; Windows 10 might only have a plurality share of enterprise systems rather than the majority share Nadella claimed. But either way, a substantial number of machines in the enterprise are currently running Windows 10.
Equally, however, it means that there's a substantial number of machines not running Windows 10. Those systems are likely to be running Windows 7. Windows 7 is due to drop out of support in January 2020. Beyond that date, Windows 7 users will either have to pay for up to three years of patches or switch to Microsoft-hosted virtual machines, which will receive the three additional years of patching at no cost.