The 10% excel sheets are exactly the problem. You can replace 90% or even 99% with either some online simpler office suite or none at all. But it will turn out there are business critical things running in 2003 format excel sheets with macros and if anything stops working then the organization stops working. The person who made these sheets and macros has retired 20 years ago and "updating it" would require making a dozen little applications to replace them which would be a huge task.

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)