The simple/cheap answer I think is to just keep the power users and legacy systems on the tools they need/want. Because that's where the pain point is that cause these projects to fail. If you aim for 100% you'll end up with 0% eventually. If you aim for 90%, you can do it.
But Microsoft know this, and would make sure that a 90% migration saves you much less than a 100% migration. Which is why this is so rare still.
(I think the number is probably 99% not 90%, but the point is the same)
SaaS and mobile has taken over many apps. We sell SaaS to business and govt clients where most end users are either on a browser or a mobile app. There is a Windows application (path dependent history) but it is almost vestigial for configuration/management.