MS could try to use its application software to force a awitch to 10...
... but it could be a double edged sword.
MS could for example make the next Office running on Windows 10 only. People on the Office 365 subscription could be forced to upgrade (this is another issue with the subscription model)
But it could be a risky move, because it could make people look for alternatives to be strangled into a system where MS dictates upgrades, not the users.